Final answer:
A subscript is a small number below an a) element symbol that represents the atomic number of an element in chemistry. Hence, option a) is correct.
Step-by-step explanation:
A subscript is a small number below an element symbol. In chemistry, the atomic number of an element is represented by a subscript to the left of its symbol.
The atomic number defines the element and indicates the number of protons in an atom of that element. For example, ₁H represents hydrogen, which has one proton, and C represents carbon, which has six protons.