Question

In the metric system, the base unit for volume is the

Answer 1

The base unit would be liter.

Answer 2

The unit of volume most often used is the liter, because it's
so handy and useful in everyday life (except in the USA, that is). 

But the liter isn't the "base unit" of volume.  The base unit is the
cubic meter, and the liter is 0.001 of that.