a.

Because the sizes of heap blocks are almost always dynamically calculated to be large enough to fit incoming data

b.

Because programmers hard-code the size of a heap block when they have variably sized incoming data

c.

Because the pointers to the next and previous heap blocks are placed either right before or right after the actual block data

d.

Because heap blocks become a problem when the programmer miscalculates the number of bytes needed to hold a particular user-supplied buffer in memory