How does slow resistance work?

The spell "Willpower" can be learned and at level 6 it provides a 50% resistance to slow and immobilization effects.

Does this mean:
A) 50% of the time you are not slowed when you otherwise would be
B) You are only slowed for 50% of the normal duration
C) Your speed is only slowed by half the amount it normally would be

I am guessing that immobilization resistance works the same way as slow, so the most likely answer would be A) if that is the case.

Follow-up question: Does the slow resistance from this ability apply to all effects that slow, such as monsters with the Freezing keyword?

Follow-up question: Does knockback resistance work the same way?

Solution 1:

The answer is A. 50% of the time the spells won't work on you at all.