The answer is
$C$
The reason is
Our ant's relative velocity to the returning ant;'s is 4 ant's per second
The explanation:
-If our ant was not moving, then 2 ant-lengths per second would pass him, and 120 ants per minute would pass him.
-But our ant is moving from A to B at a velocity of 2 ant-lengths per second.
-Returning ants are moving from B to A at a velocity of 2 ant-lengths per second.
-Therefore, our ant is moving relative to the returning ants at 4 ant-lengths per second (2+2).
-60 seconds per minute * 4 ant-lengths per second = 240 ant-lengths per minute.