What would happen if gametes were not haploid?
a. the offspring would have half the number of chromosomes it is supposed to have.
b. the offspring would have double the number of chromosomes it is supposed to have.
c. the offspring would be genetically identical to it's parent.
d. the offspring would be genetically different from it's parent.

The answer is b. The offspring would have double the number of chromosomes it is supposed to have.

Somatic cells in the organism are diploid (2n). This means they have n number of chromosomes in duplicate. Gametes (egg cells and sperm cells) are haploid cells (n) which mean they have only n number of chromosomes. After haploid egg cell (n) fuse with haploid sperm cell (n) fuse, a diploid zygote (2n) is formed:
n + n = 2n
So, again, somatic cells of the newly formed organism are diploid (2n).

Now, imagine a situation that gametes are not haploid (n) but diploid (2n). After fusion of diploid egg cell (2n) and diploid sperm cell (2n), a tetraploid zygote (4n) would be formed:
2n + 2n = 4n

This is twice as many chromosomes than the normal offspring will have.
Therefore, if gametes were not haploid, the offspring would have double the number of chromosomes it is supposed to have.