Step 1: Understanding female sex hormones.
Estrogen is a primary female sex hormone that regulates reproductive and sexual development in females. It is involved in the menstrual cycle, pregnancy, and puberty.
Step 2: Analyzing the options.
(A) Stilbestrol: Stilbestrol is a synthetic estrogen, but it is not naturally occurring in females.
(B) Testosterone: Testosterone is a male sex hormone, although it is also present in small amounts in females.
(C) Estrogen: Correct — Estrogen is the primary female sex hormone responsible for regulating many functions in females.
(D) Benzestrol: Benzestrol is a synthetic compound similar to estrogen but not a natural female sex hormone.
Step 3: Conclusion.
The correct answer is (C) Estrogen as it is the primary female sex hormone.