The marking is incorrectly red-lining the 's' on " grands-" in < grands-parents >. This had occurred in another lesson previously as well and was subsequently reported to have been corrected.
The use of < l'a réussi > for "passed it" is also being red-lined incorrectly, as also recently noted by James. As < l'a obtenu > is not accepted either, this suggests only 1 correct way to say passed an exam in French.
The choice of imparfait or passé composé for devoir in "she had to work hard" would be made clearer if the rest of the sentence was on the same screen - the final screen presented would then be 'she had to work hard but she passed it'.
(Per the lesson on devoir in imparfait or passé composé - both can be translated into English as 'had to', and it is the 'certainty of outcome/completion' that defines the choice.
