I always found these kind of puzzles silly. There are an infinite number of justifications one could make -- and what you're really trying to do is predict the kind of justification a test-maker would deem sufficiently intelligent-sounding so as to warrant consideration.
That being said, here are my thoughts:
Answer 2. All blocks have to have at least one pair. A pair can either mean two white pieces adjacent to each other and in the same direction, or a single black piece anywhere in the same direction. We can rule out 1, 5, and 6 because they either don't have adjacent white pieces, or they have more than one black piece.
Answer 3. Pretty straightforward. You need a black block in every possible horizontal configuration to make a grouping of 'four.'
Answer 6. It's hard for me to fathom why any combination of pattern ideas would yield a completely distinct block from one already present.
The most obvious answer would be half black (on top) and half white. Given that such a block is not present, perhaps the answer is as silly as "pick a block already present in the above diagram."
Edit: Fixed spoiler