The commonest cause of primary Amenorrhoea is:: (A) Genital tuberculosis, (B) Ovarian dysgenesis, (C) Mullerian duct anomalies, (D) Hypothyroidism
Answer is B. Most common cause of Primary Amenorrhea = Gonadal dysgenesis/ovarian dysgenesis
2nd most common cause of Primary Amenorrhea = Mullerian agenesis (Mayer Rokitansky Kuster Hauser Syndrome) 

“Mullerian agenesis is a relatively common cause of primary amenorrhea, more frequent than congenital androgen insensitivity and second only to gonadal dysgenesis”

 3rd most common cause is testicular feminizing syndrome.