Translation:That is their doctor.
“他” = "he/him" so ”他们“ = "they” (males). "她” = "she/her" so “她们” = "they" (females). "他们“ would be used for a group of males and females. When translating "they/them" and the context doesn't identify whether "they/them" refers to men or women, both "他们” and "她们“ should be accepted.
As I understand it, "ge" is a measure word or classifier for nouns. In 那个是 the 个 is not followed by a noun. To use 个 in this sentence you could say 那个男人是他们的医生，or, 那个女人是他们的医生 - i.e. That man is their doctor, or That woman is their doctor. After posting this message yesterday, the next question had 那个 plus a verb. So more checking in my dictionary. It looks like 那 would be "that" and 那个 would be "that one".
In Chinese, both "he" and "her" are pronounced "Tā" but spelled differently (他 and 她 respectfully). Because this was a "Type what you hear" question with no context, both 他 and 她 should also be accepted for individuals who like to type out their answers rather than use the word bank. In this case, 那是他们的医生 (Nà shì tāmen de yīshēng) should be accepted.