The answer is B. secondary spermatocytes.
A diploid organism (2n) has 12 chromosomes (2n = 12) in its germinal cells. Spermatogenesis actually consists of mitosis and meiosis (the reduction division). It starts from spermatogonia (germinal cells) that are diploid. Through the mitosis, they divide into identical primary spermatocytes. As they are the result of mitosis, they are diploid (2n), too. However, primary spermatocytes undergo meiosis. After meiosis I secondary spermatocytes are formed and they are the first that are haploid (n) and have 6 chromosomes.