Select the parameter that needs to be constant for Charles law:
A. Pressure
B. Temperature
C. Volume
D. Surface area

Hint:Identifying the parameter that needs to be constant for Charles law will be easier if we understand the concept of Charles law in detail and along with that if we express it in mathematical form.

Complete answer:
Charles law states that the volume of a fixed amount of dry gas at constant pressure is directly proportional to its absolute temperature. In other words, the law states that the gases tend to expand when heated. That is, the Kelvin temperature and the volume will be directly proportional.
\[V\propto T\],
where $V$ is volume and $T$ is temperature.
$\dfrac{V}{T}=k \\
\Rightarrow V=kT $
Therefore, it is the pressure that is constant for Charles law. Directly proportional means any change made in one of them will result in the change in the other quantity through direct variation.For the same gas, at a different temperature, and volume, the non-zero constant $k$ will be equal.
$\dfrac{{{V}_{1}}}{{{T}_{1}}}=\dfrac{{{V}_{2}}}{{{T}_{2}}}$
$\therefore {{V}_{1}}{{T}_{2}}={{V}_{2}}{{T}_{1}}$

This equation is a clear example that shows that the volume increases as the temperature of the gas increases. ${{V}_{1}}$ and ${{V}_{2}}$ are the initial and final volumes and ${{T}_{1}}$ and ${{T}_{2}}$ are the initial and final temperature respectively. The temperature is measured in terms of Kelvin. Therefore, the parameter which needs to be constant according to Charles law is Pressure.

Hence, the correct answer is option A.

Note:The name of the law is Charles law as it is named after a scientist named Jacques Charles. He formulated and published the law in the $18$ th century around the year $1780$. This law is widely used in the kinetic theory of gases.