The correct option is C 12 cells of antheridium + 1 prothallial cell
Microspores of Selaginella are haploid in nature. Each microspore divides by mitosis division and forms a small lens shaped prothallial cell and a large antheridial cell. Subsequent division only occur in antheridial cell which result of a 12 celled antheridium in which outer 8 cells form jacket of antheridium and inner 4 cells form primary androgonal cells. Thus mature microgametophyte contains 12 antheridial cells and 1 prothallial cell.