**Core Concept**
Tubocurare is a non-depolarizing neuromuscular blocker that acts by competing with acetylcholine for **nicotinic receptors** at the neuromuscular junction, thereby inhibiting muscle contraction. This action is based on the principle of competitive inhibition of **acetylcholine receptors**. The effect of tubocurare is first seen in the muscles that are rich in **nicotinic receptors**.

**Why the Correct Answer is Right**
The correct answer is related to the fact that tubocurare affects the muscles that have a high density of **nicotinic receptors**. The small, rapidly moving muscles are typically the first to be affected by non-depolarizing neuromuscular blockers like tubocurare. This is due to the higher density of **nicotinic receptors** in these muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the effect of tubocurare is not typically first seen in this muscle group.
**Option B:** This option is incorrect because, although it is a muscle group, it is not the first to be affected by tubocurare.
**Option C:** This option is incorrect because the muscle group is not the primary site of action for tubocurare.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the order of muscle relaxation by non-depolarizing neuromuscular blockers like tubocurare typically follows the order of small, rapidly moving muscles to larger muscle groups.

**Correct Answer:** D. Extraocular muscles.