The mass number of nucleus is:
A. Sometimes equal to its atomic number
B. Sometimes less than and sometimes more than its atomic number
C. Always less than its atomic number
D. Always more than its atomic number

Hint- In order to deal with this question first we will talk about the mass number then we will proceed further by understanding the term nucleus and according to it we will find our required answer.

Complete step-by-step answer:
The cumulative number of protons and neutrons (together known as nucleons) in an atomic nucleus is the mass number (symbol A, from the German term Atomgewicht [atomic weight]), also called atomic mass number or nucleon number..
It is approximately equal to the atomic mass measured in atomic mass units. Since protons and neutrons are both baryons, the mass number $A$ is identical with the baryon number B as of the nucleus as of the whole atom or ion. For each varying isotope of a chemical element, the mass number is different. Hence, the difference between the mass number and the atomic number $Z$ gives the number of neutrons $\left( N \right)$ in a given nucleus: $N = A - Z$.
Hence, the mass number of a nucleus is sometimes equal to its atomic number.
Therefore the correct answer is option A.

Note- A chemical element's atomic number, or proton number, is the number of protons present in the nucleus of each atom of that element. A chemical element is uniquely defined by atomic number. It is equal to nucleus charge number. The atomic number of an uncharged atom is equal to the number of electrons, too. The sum of the atomic number Z and the number of neutrons N establish an atom's mass number A.