The first ionisation potential of calcium is greater than that of potassium because for calcium

We know that Atomic number of Calcium is 20 and the atomic number of potassium is 19.

And, we also know that as we move along the period atomic size decreases and nuclear charge increases.

We know the higher the nuclear charge, the greater the ionisation energy is required to remove the electron that is why the first ionisation potential of calcium is greater than that of potassium.

Hence, option(a) is the answer.