I'm not sure why चाचा is wrong, or why it is an option along with मामा. Are they both not words for uncle? I see nothing in the sentence which tells us if it is a maternal or paternal uncle.
Any of मामा, चाचा, मौसा, and फूफा should be accepted. I hope you used the report button.
I did. I'm sure they'll figure it out. I'm just happy that Hindi is finally available.
Why is जूलिया का चाचा बीमार है not accepted?
Because that would be a very rude way to say that. When referring to elders, honorific plurals are used.