Final answer:
The notation "Za" indicates the atomic number of an element, which represents the number of protons in its nucleus. Hence the correct answer is option A
Step-by-step explanation:
The notation "Za" refers to the atomic number (A) of an element. The atomic number represents the number of protons in the nucleus of an atom. It is a unique characteristic of each element and determines its position in the periodic table. In the notation, the symbol 'Z' represents the number of protons, while 'a' represents the chemical symbol of the element.