## Core Concept
The question tests the knowledge of drug administration routes, specifically focusing on oral administration. Various drugs have different routes of administration based on their pharmacokinetics, pharmacodynamics, and therapeutic goals.

## Why the Correct Answer is Right
Insulin is primarily administered via injection (subcutaneously) because it is a peptide hormone that would be digested in the gastrointestinal tract if taken orally, rendering it ineffective. This makes **D. Insulin** the correct answer.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is not specified, but if we consider common drugs used orally, many are indeed administered through the oral route for various therapeutic effects.
* **Option B:** Similarly, this option is unspecified but could potentially represent a drug commonly given orally.
* **Option C:** This option is also unspecified but might represent another drug that is administered orally.

## Clinical Pearl / High-Yield Fact
A key point to remember is that peptide hormones like **insulin** and **growth hormone** are not effective orally due to digestion in the gastrointestinal tract. This is why they are administered via injection.

## Correct Answer: D. Insulin