The goons ____ the unsuspecting victims when the latter walked into the dark alley.

The phrasal verb 'set upon' means to attack someone suddenly and violently. In this context, the goons attacked the unsuspecting victims in the dark alley — so 'set upon' is the correct choice. 'Set up' means to frame or establish, 'set down' means to write down or land, and 'set along' is not a standard phrasal verb. Hence option (a) is correct.