Imagine that Mister Smith and his wife are standing in front of you. They have come with their only child. Let us call him John. Now you ask Mister Smith and his wife (two people) if their son like fruit.
'Synem' is in the instrumental case. Since the son is the subject of the sentence, you must use the base form (nominative) 'syn'. Although I'm surprised that it showed you 'synek' (diminutive), because it's not the main answer, just an accepted one.
It seems that what the grading algorithm suggests tends to change quite often, but in a way "synek" is the closest correct answer to what Kristine wrote, I can imagine the algorithm 'thinking' that it was just a typo rather than the wrong form.