"He was planting potatoes."
Translation:Bhí sé ag cur prátaí.
I got caught on this one too. But it's not a rejection of fataí but because the sentence contained sí instead of sé.
Not so. Two of the choices were:
Bhí sé ag cur prátaí (correct)
Bhí sí ag cur fataí (incorrect) because the subject is sí instead of sé.
Had it been: Bhí sé ag cur fataí it would most likely have been accepted.