**Core Concept**
The question is testing the student's understanding of the pathophysiology of hirsutism, a condition characterized by excessive hair growth in women, often due to an excess of androgens. The normal ovaries on USG suggest that the excess androgens are not produced by the ovaries, pointing towards an external source.

**Why the Correct Answer is Right**
The most likely diagnosis is Congenital Adrenal Hyperplasia (CAH) due to 21-Hydroxylase deficiency. This enzyme deficiency leads to an accumulation of 17-hydroxyprogesterone, which is then converted to androgens, resulting in an excess of free testosterone. The normal ovaries on USG rule out ovarian causes of hirsutism, such as polycystic ovary syndrome (PCOS). The excess facial hair and oligomenorrhea are symptoms of hyperandrogenism.

**Why Each Wrong Option is Incorrect**
* **Option A:** Polycystic Ovary Syndrome (PCOS) is a common cause of hirsutism in women, but the normal ovaries on USG make this diagnosis less likely. PCOS is typically characterized by polycystic ovaries on ultrasound.
* **Option B:** Cushing's syndrome is a cause of hirsutism due to excess cortisol, which can be converted to androgens. However, the normal ovaries on USG and the lack of other symptoms of Cushing's syndrome make this diagnosis less likely.
* **Option C:** Androgen-producing tumors of the ovary are a rare cause of hirsutism. The normal ovaries on USG make this diagnosis highly unlikely.

**Clinical Pearl / High-Yield Fact**
CAH due to 21-Hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia, accounting for approximately 90% of cases. It is an autosomal recessive disorder, and affected individuals often present with salt wasting, hypotension, and hyperkalemia in infancy.

**Correct Answer:** C.