Ans. is 'a' Serial ultrasound (Ref. Shaws, 13/e, p 305 (12/e, p. 242)).In patients with infertility and being treated with clomiphene monitoring of Graafian follicle is required to check its maturity and to detect imminent ovulation.This monitoring is done with serial ultrasound starting from day 10th to day 16th.It is non-invasive, safe and accurate method and is the standard investigation to detect ovulation.