**Core Concept:** The commonest site of fertilization occurs in the fallopian tubes, specifically in the ampulla or isthmus regions. This is where the ovary releases the mature egg and sperm enter the fallopian tube during sexual intercourse.

**Why the Correct Answer is Right:** The correct answer, **Option C: Ampulla of Fallopian tube**, is the most common site of fertilization due to the following reasons:

1. **Ampulla region:** The ampulla is the narrow, V-shaped part of the fallopian tube where the oviduct (fimbria) meets the uterus (cornua). It is the narrowest part of the tube, and the sperm must pass through this narrow space to reach the oocyte (egg).
2. **Sperm storage and motility:** The sperm are stored in the ampulla after entering the fallopian tube, allowing them to accumulate and swim towards the oocyte. The ampulla also has a higher concentration of prostaglandins, which enhance sperm motility and prepare the uterus for implantation.

**Why Each Wrong Option is Incorrect:**

1. **Option A (Isthmus):** The isthmus is the middle part of the fallopian tube, connecting the ampulla to the uterus. While fertilization can occur in the isthmus, it is less common than in the ampulla.
2. **Option B (Oviduct):** The oviduct is the incorrect term for the fallopian tube. The correct term should be fallopian tube.
3. **Option D (Oviduct):** This is another incorrect term for the fallopian tube.

**Clinical Pearl:** Understanding the common site of fertilization helps in understanding the reproductive process and can aid in clinical scenarios like evaluating infertility and assessing the cause of unexplained infertility.