According to the text, spermatogonia and primary spermatocytes are diploid cells that undergo meiosis. The first meiotic division (reduction division) results in the formation of haploid secondary spermatocytes or secondary oocytes and the first polar body. The secondary spermatocytes/oocytes undergo the second meiotic division to form haploid spermatids/ovum and the second polar body. Therefore, the group containing only haploid cells is 'Secondary spermatocyte, First polar body, Ovum'. Primary oocytes and primary spermatocytes are diploid.