"My dad is an engineer."
Translation:Mon papa est ingénieur.
I'd say the rule of thumb is to not use articles when talking about someone being something
I agree with duolingo on this one. "Mon papa est ingénieur" is correct. You don't use "un" or "une" when referring to someone being "an engineer" or "a teacher" or "a whatever" where whatever is that person's nationality, religion or profession.
Out of the three translations to choose from, the one that is right among them seems to be wrong... I had to choose, "Mon pere est un ingenieur," but I think it's supposed to be, "Mon pere est ingenieur' Is it? Basically I think the 'un' is not supposed to be there at all.
It's true that we rarely put any article before a job. So i'd say the most correct answer should be "Mon père est ingénieur". Put the other one is not wrong. Just less common.
Keeping to the same level of informality, "dad" is "papa" and "father" is "père".