The word 'voyage' means 'journey'. This means that the subjects were travelling. The correct answer must justify that the subjects were travelling.
Option A is incorrect. It indicates that the subjects were at school. Thus, this does not justify that the subjects were travelling.
Option B is incorrect. Terrace is wrong because it not only fails to justify the travelling of the subjects but also makes the sentence grammatically incorrect.
Option C is correct. Here 'at sea' means 'travelling on sea'. Thus, it indicates that the subjects were travelling and so, it is the correct option.
Option D is incorrect. It indicates that the subjects were at home and not travelling. So, it is the wrong option.