Which word in the following sentence is modified by the adjectival clause: "a video game which spans the ages is pac-man? "
a. game
b. which
c. ages
d. pac-man

The adjectival phrase is the phrase that functions as an adjective: that is, it has the same function that an adjective would. 

The adjectival phrase here is "which spans the ages". 

It gives some information about the video game (compare an adjective that could be used: a good video game). 

"video" can actually be understood as an adjective to "game", in which case the adjective phrase modifies a. game.