Which contains more molecules: a mole of hydrogen or a mole of chlorine

Neither.

Step-by-step explanation:

A mole of particles contains Avogadro's number of particles. Thus,

A mole of hydrogen contains 6.023 * 10²³ hydrogen molecules.

A mole of chlorine contains 6.023 * 10²³ chlorine molecules.

A mole of hydrogen and a mole of chlorine contain the same number of molecules.

b is answer good luck.