What is the key endogenous variable in the quantity theory? Explain the effect on this key variable

Question:

What is the key endogenous variable in the quantity theory? Explain the effect on this key variable of the following changes:
(a) The money supply is doubled.
(b) The velocity of money increases by 10%.
(c) Real GDP rises by 2%.
(d) The money supply increases by 3% while real GDP rises by 3% at the same time.