Trichomonas vaginalis causes sexually transmitted disease like trichomoniasis which results in leucorrhoea (or leukorrhea ). It is a white, sticky, foul-smelling vaginal discharge which refers to the presence of polymorphonuclear leukocytes (PMNLs) in vaginal fluid.
Leishmania, a genus of trypanosomes, infect man, dog, cattle, horse, sheep etc. and cause serious diseases collectively known as Leishmaniasis. Trypanosoma species causes a disease called as trypanosomiasis (sleeping sickness or chaga's disease).
Giardia lamblia causes parasitic disease of the small intestine called as giardiasis (beaver fever).
Thus, the correct answer is option A.