A is the correct answer.
This is a special case of the Ideal Gas Equation called Boyle's Law.
In a closed system at one temperature (T = constant), the number of moles of gas (n) are constant, and the Universal Gas Constant remains the same, so nRT would be a constant value.
Therefore, P1V1 = nRT = P2V2
In other words, as pressure increases, volume decreases; and as pressure decreases, volume increases.
38 years in chemistry