Earliest of fetal hea can be detected at:
When the question says - Cardiac activity is earliest detected at it means it's about TVS (Transvaginal sonography),which is seen at 6 weeks. Ref: Callen 4e pg 119,120.
6-6.5 weeks
6.5-7 weeks
7.1-7.5 weeks
8 weeks
General obstetrics