**Core Concept**
The question is testing the student's knowledge of orally active hormones, which are hormones that can be taken orally and still retain their biological activity. This concept is crucial in endocrinology, where the route of administration can affect the efficacy and bioavailability of hormones.

**Why the Correct Answer is Right**
The correct answer is a hormone that can be taken orally and is not significantly metabolized by the liver or gastrointestinal enzymes. One such hormone is **Levothyroxine (T4)**, which is a thyroid hormone replacement therapy. When taken orally, T4 is absorbed from the gut and then converted to its active form, **Triiodothyronine (T3)**, in the liver and peripheral tissues. This conversion is essential for T4's biological activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify an orally active hormone.
**Option B:** This option is incorrect because it is not a hormone that is commonly taken orally.
**Option C:** This option is incorrect because it is not a hormone that is known for its oral bioavailability.

**Clinical Pearl / High-Yield Fact**
When prescribing oral thyroid hormone replacement therapy, it is essential to note that the patient's thyroid function should be monitored regularly, as the conversion of T4 to T3 can be affected by factors such as liver disease, malnutrition, and certain medications.

**Correct Answer: D. Levothyroxine (T4)**