The relation between atomic number $\left( Z \right)$ and atomic mass $\left( A \right)$ for the light element is given by:
A) $A = Z$
B) $A = 2Z$
C) $2A = Z$
D) $A = 4Z$

Hint: The light elements are the elements in the periodic table that have an atomic number lower than twenty. This information can be used to solve the above question as the first twenty elements are known to everyone. One can also relate their knowledge in the given options to solve the question.

Complete step by step answer:
1) The atomic number of an element can be defined as the number of protons present in the nucleus of that element. For example, $H$ has only one proton hence, it has the atomic number $1$ and it is the lightest element on earth.

2) The mass number of an element can be defined as the sum of the number of protons and neutrons present in the nucleus of that element. For example, $H$ has atomic mass $1$ as it has only one proton and zero neutrons in its nucleus.

3) The light elements are the first $20$ elements of the periodic table. They are called light elements because of their low atomic number and low atomic mass.

4) Usually, the light elements follow the one general formula for their atomic mass, which is exactly double the number of the atomic number of that element. Hence, the general formula can be written as follows,
$A = 2Z$
The $2Z$ is used because the number of neutrons for light elements is always equal to the number of protons present in that element.

5) Therefore, the relation between atomic number $\left( Z \right)$ and atomic mass $\left( A \right)$ for the light element is given by the formula $A = 2Z$ which shows option B as the correct choice.

And hence option B is a correct choice.

Note:
The number of neutrons is not always equal to the number of protons which is the only case of light elements possible. For higher elements, the calculation of the number of neutrons is done by subtracting the number of protons of that element from the atomic mass number of that element.