Will knows that the gravitational pull of mars is less than the gravitational pull of earth. when he lands on mars, his mass will be greater than or equal to 75kilograms?

Mass of an object/person remains same, doesn't depend on gravity, so if his mass is 75 Kg somewhere, then, it would remain same everywhere. Only Weight of a person changes with gravity.

In short, Your Answer would be "Equal to 75 Kg"

Hope this helps!