As it's a homework question here is a hint:

Imagine Ant A is stationary (it could be)...

The let's say B moves a small distance, along the line AB.  How would this change distance AB? 

Then, what would this mean for how distance AC has to change, and in what direction must C move - then what would that mean for what $V_C$ has to be (in terms of $V_B$)?

This seems to rule out three of the options.