According to Charles law,
(a). At constant pressure, volume of gas is proportional to its absolute temperature.
(b). At constant pressure, the volume of gas is not proportional to its absolute temperature.
(c). At constant gauge pressure, the molecular volume of a gas is proportional to its absolute temperature.
(d). At constant volume, the absolute pressure is proportional to absolute temperature.

Hint: Charles law deals with volume and temperature. While Boyle’s law deals with pressure and volume and Dalton’s law deals with total pressure and partial pressure. Use the ideal gas equation to get a relation of Charles law.

Complete step-by-step answer:

According to ideal gas equation,
PV = nRT
Where,
P = pressure
V = volume
N = number of moles
R = universal gas constant, whose value is $8.314mo{{l}^{-1}}{{K}^{-1}}$
T = absolute temperature
Therefore, we can write,
$V=(\dfrac{nR}{P})T$
From the above equation we can conclude that volume is directly proportional to temperature.
Mathematically it is written as,
$V\alpha T$
And above equation is nothing but Charles law which states that “At a fixed pressure and for given mass of a gas, the volume of a gas is proportional to its absolute temperature”
So as we can see in options, option A is correct because the meaning of its statement is the same as that of Charles' law.
Hence, option (a) is correct

Note: Do not get confused between Boyle’s law and Charles law. Boyle’s law states that “At constant temperature, the pressure exerted by a fixed mass of a gas is inversely proportional to volume occupied by gas.