reacts with CaCO3 at the faste

Ans: 1M H2SO4 (assume the solutions are in same volume)
There is no reaction between CaCO3 and KOH.
Consider 1M HNO3 and 1M H2SO4, the no. of moles of H+ in H2SO4 doubles that of the HNO3. Thus, there are higher chance for CaCO3 to collide with H+ in H2SO4 rather than in HNO3. So reaction rate is the fastest for H2SO4.

2011-04-06 15:49:55 補充：
If formation of insoluble CaSO4 have also required to be considered, I think experiments have to be carried out to compare the rate.. because there are two factors affecting the rate here - one is concentration and other is solubility, which makes the question more complicated....

have you considered the formation of insoluble CaSO4 ?