I suspect "They study sometimes in the morning" should not only be accepted, but is better grammar.
It should certainly be accepted but I am dubious it is actually better syntax than the given answer - there is nothing grammatically wrong with either version, or indeed with the third one that has been given.
As a 'correct' answer is given: "They sometimes prep in the morning" - Why "prep"? 'Sometimes' can be used on several places in the sentence - "Sometimes they study in the morning", "They sometimes study in the morning" or (as mentioned by 'Mstanding') "They study sometimes in the morning".
Yes it estudian is sometinmes (a veces!) best translated to English as tbey are studying. But on this occasion the use of "a veces" makes the simple present, they study, very much apptopriate in tbe English translation.