**Core Concept**
Tubocurarine is a non-depolarizing neuromuscular blocking agent that competes with acetylcholine for receptors at the neuromuscular junction, leading to muscle paralysis. The drug's mechanism of action is based on its ability to block nicotinic acetylcholine receptors (nAChRs) on the muscle endplate.

**Why the Correct Answer is Right**
Tubocurarine is a competitive antagonist that binds to the nAChR at a site distinct from the acetylcholine binding site. It has a higher affinity for the receptor in the diaphragm than in other muscles, making it the first muscle to be affected. This is due to the unique physiological characteristics of the diaphragm, which has a higher concentration of nAChRs per unit area compared to other muscles.

**Why Each Wrong Option is Incorrect**
* **Option A:** This muscle is not the first to be affected by tubocurarine. The drug's action is not specific to this muscle.
* **Option B:** While tubocurarine does affect this muscle, it is not the first to be relaxed. The diaphragm has a higher receptor density, making it more susceptible to the drug's effects.
* **Option C:** This muscle is not typically affected first by tubocurarine. The drug's action is more pronounced in muscles with a higher receptor density, such as the diaphragm.

**Clinical Pearl / High-Yield Fact**
Tubocurarine's higher affinity for the diaphragm's nAChRs makes it a useful agent for surgical muscle relaxation, particularly in procedures where diaphragmatic movement is a concern.

**Correct Answer:** B. The extraocular muscles.