**Core Concept**
The SGLT-1 cotransporter is a sodium-glucose linked transporter that plays a crucial role in the absorption of glucose in the gastrointestinal tract (GIT). It is a type of secondary active transporter that uses the electrochemical gradient of sodium ions to transport glucose against its concentration gradient.

**Why the Correct Answer is Right**
The SGLT-1 cotransporter is responsible for the absorption of glucose from the luminal contents of the small intestine into the enterocytes. This process involves the cotransport of one glucose molecule and one sodium ion across the apical membrane of the enterocytes. The glucose is then transported into the bloodstream via facilitated diffusion through the GLUT2 transporter. The correct answer is **Glucose** because it is the primary substrate for the SGLT-1 cotransporter.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glutamine is not the primary substrate for the SGLT-1 cotransporter. While glutamine is an important energy source for enterocytes, it is transported through a different mechanism.
* **Option B:** Amino acids are not the primary substrates for the SGLT-1 cotransporter. Amino acids are transported through different transporter proteins, such as the PEPT1 and PEPT2 transporters.
* **Option D:** Sodium ions are not the primary substrate for the SGLT-1 cotransporter, although they are required for the cotransport mechanism. The sodium ions are simply used to drive the transport of glucose against its concentration gradient.

**Clinical Pearl / High-Yield Fact**
The SGLT-1 cotransporter is an important target for the treatment of glucose malabsorption disorders, such as celiac disease and tropical sprue. Inhibitors of the SGLT-1 cotransporter, such as acarbose, can be used to reduce glucose absorption in these conditions.

**Correct Answer: A. Glucose**