**Core Concept**
The femoral triangle is an anatomical region in the lower limb, located in the anterior (front) aspect of the thigh. It is a clinically important area due to the presence of vital structures such as the femoral artery and vein, which are essential for lower limb perfusion. The boundaries of the femoral triangle are formed by muscles and fascia, which provide a framework for the identification of its contents.

**Why the Correct Answer is Right**
The base of the femoral triangle is formed by the inguinal ligament, which is a fibrous band that connects the pubic bone to the anterior superior iliac spine (ASIS). This ligament serves as the inferior boundary of the femoral triangle, separating it from the medial (inner) aspect of the thigh. The inguinal ligament is a tough, fibrous structure that provides support to the abdominal wall and helps to maintain the position of the femoral triangle.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the base of the femoral triangle is not formed by the sartorius muscle, which is one of the muscles that form the medial boundary of the triangle.
* **Option B:** This option is incorrect because the pectineal ligament does not form the base of the femoral triangle, although it does provide a connection between the pubic bone and the femoral artery.
* **Option C:** This option is incorrect because the adductor longus muscle is one of the muscles that form the medial boundary of the femoral triangle, but it does not form the base.

**Clinical Pearl / High-Yield Fact**
The boundaries of the femoral triangle are often remembered using the mnemonic "FAN", which stands for femoral artery, nerve, and vein. This mnemonic helps to identify the key structures within the triangle and their relationships to each other.

**Correct Answer: D. Inguinal ligament**