The correct sentence is the falling snow when the students are walking to their homes from class. This was clearly given when the snow begins to fall.
The other sentences are grammatically wrong because “watched” the snow “fell”. The past tense does not come on this manner to form a grammatically correct sentence.
Thus, the options (d), (c) and (a) are incorrect.
The correct sentence is the falling snow when the students are walking to their homes from class. This was clearly given when the snow begins to fall.
Therefore, the correct option is – a. Walking home from class, the students watched the snow begin to fall.