Well the trivial answer is zero, since there is indeed a "zero vector". Assuming you aren't allowed to use the zero vector you would need at least two. They would be antiparallel and of equal magnitude. (That is be pointing in opposite directions and have the same length)
I am pretty sure that the only statement which is true for particles of the medium of an earthquake P-wave is being shown in the option : b)vibrate parallel to the wave, forming compressions and rarefactions. As you know, it can be formed in two ways : from alternating compressions and rarefactions or primary wave. I bet you will agree with me.
The answer is option A, i think but i am not sure
