The correct option is
C Monocystis
The fusion of similar motile gametes is known as isogamous. Monocystis is an endoparasite in the seminal vesicle and coelom of earthworms. Fusing gametes do not differ morphologically but may be different physiologically. The isogamous gametes fuses to form zygote which develops into sporoblast, sporocyst, trophozoite and finally, to young trophozoite.
So, the correct answer is option B.