The dropdown for उसका gives he/she/it/that has as the first option. Therefore I chose He has a house in America as the translation. Is this wrong? If so, why, and please fix the drop-down for this. Especially when this is the first sentence I learn on it.
I have made a similar choice before I noticed that the word Duo wants to see at the beginning of the sentence is always the capitalized word box.
That's not a very good system - if a word can mean both 'his' and 'her,' and they don't provide context (via a preceding sentence in the same exercise about the individual), it should (IMO) accept all correct answers. I used 'their,' as it's the grammatically correct gender-neutral third person singular pronoun in English, and would also have fit.
You don't. उसका/इसका/उसकी/इसकी/उसके/इसके always correspond with the gender of the object(s) behind possessed, never with the gender of the person.
For example, उसका नाम क्या है? can mean both "What is her name?" or "What is his name?"
Because Hindi uses postpositions, not prepositions. In other words, in English words like "in," "at," "for," "from," etc. go before the noun but in Hindi "में," "पर," "के लिए," "से," etc. go after the noun.
"America" is also short for "The United States of America," so depending on where in the world you are, you'll hear people say "from the US", "from the US of A", "from the United States", or "from America" all meaning the same thing.
I've found it only an issue when in Central or South America.