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.<br/><br/>
>! -But our ant is moving from A to B at a velocity of 2 ant-lengths per second.<br/><br/>
>! -Returning ants are moving from B to A at a velocity of 2 ant-lengths per second.<br/><br/>
>! -Therefore, our ant is moving relative to the returning ants at **4 ant-lengths per second** (2+2).<br/><br/>
>! -60 seconds per minute * 4 ant-lengths per second = 240 ant-lengths per minute.