Translation:He is the engineer, she is the architect.
actually, the definite article is not used when talking about professions in general in English. It should be at least allowed to write He is an engineer....
Since the program allows flexible translations of so many things where the literal translation does not conform to normal usage, I would think this would be easily accepted. Because otherwise you end up focusing more on learning how to answer questions rather than learning the answers for questions.
"He is an engineer" would have a different translation in Italian ("Lui è [un] ingegnere."). This also has a different meaning (being an engineer is his job, rather than him being the specific engineer). This is perfectly normal usage. Think of it as the answer to the question, "Who is working on the project?" "He is the engineer..." "An engineer" and "the engineer" aren't interchangeable in either the English or the Italian.
Most professional nouns seem to be gendered (dottore/dottoressa, scrittore/scrittrice etc.), so why not this one? Is it just one of those arbitrary things?
A bit of digging suggests that although opinion is divided, several authorities do recognise the feminine form of architetto: http://uk.answers.yahoo.com/question/index?qid=20110115025200AAI9AeI
Teta in portuguese also means tit, yet we do not call a woman an "arquiteto", but an "arquiteta". If we did so we would be calling her a man. I think the original sentence is wrong, it really should be "architetta", since the pronoun "lei" indicates that the gender of the person we are speaking about is feminine.
apart from this, there are lot of other professional nouns which are traditionally used only in the male form (sindaco,avvocato, assessore). Nowadays somebody starts questioning this status quo. (http://www.lastampa.it/2012/05/19/societa/e-l-ora-della-sindaca-e-dell-architetta-ly2hjaWIyeUiMMUvVwdwqJ/pagina.html)
The adjective (= qualifier) is in the same gender and number as the noun. So lei è un architetto creativo even if we are referring to a lady. The other way around is also true, when you have a noun whose gender is only feminine (like la guardia): la guardia è attenta even if it refers to a man.
There is actually quite a debate in Italy about feminine forms for some professions that were typically male. While pairs such as maestro/maestra, dottore/dottoressa, infermiere/infermiera are normal, othes are not so widespread. For example: ministro/ministra, sindaco/sindaca/sindachessa are fighting to earn their place in everyday language. architetta is not wrong, yet not so common, especially because of how it ends.
In my opinion, Duolingo should take a vanguardist position and translate architect (f) as "l'architetta". It's not our problem if people in Italy have an issue with the word tits. Other languages, as portuguese and spanish, have already overcome this. If you have a problem with the word tits, you're the problem. Grow up. Seriously.
We accept "l'architetta" if you are translating this sentence to Italian, but "l'architetto" is still the most common feminine form, so it's good to learn it. Every language has adapted to women entering traditionally male professions in different ways. French also uses male forms to refer to females in some professions, as does Spanish, so Italian is by no means an exception. Will this change in the future? Perhaps. But here we have to go with how the language works now.
Yay Duo! * when your profession actually comes up first in the lesson *. (Followed by the clown o.0 Oh well).
My two cents for the debate of whether the use of "the" is appropriate here: I'd say yes, it is. Examples such as those that mmseiple and PeterSoda have already given do actually come up quite often IRL.