How is the combined gas law modified to form the ideal gas law? a) each side of the equations is divided by the number of moles b) the temperature of the gas is stipulated to be absolute zero c) each side of the equation is multiplied by air pressure at sea level d) the volume of each gas particle is added to the equation

The answer is A) each side of the equations is divided by the number of moles. 
The combined gas law equation
     P1V1/T1 = P2V2/T2
becomes
     P1V1/n1T1 = P2V2/n2T2
with the addition of Avogadro's law relating volume and number of moles of a gas.This modification is when the number of moles of gas in a sample is allowed to change in addition to pressure, temperature and volume. This also means that pressure multiplied by the volume and divided by the product of the number of moles and temperature is a constant:
     PV/nT = constant
which if written in the form PV = nRT is the ideal gas law equation where R is the gas constant.