An atom of calcium has $20$ protons, how many electrons does it have?

The amount of protons in an element or atom determines its atomic number, or how many electrons are present in that particular atom or element. Now that you know this, you can simply respond to the provided statement in the appropriate manner.

The number of electrons present in a neutral atom that represent its atomic number is always equal to the number of protons present in the nucleus so that the atom as a whole is found to be electrically neutral.

We know that there are 20 protons in the calcium atom, thus, the number of electrons in the calcium atom is also 20, and hence its atomic number is 20.

Hence option (D) is correct.