**Core Concept**
Non-depolarizing muscle relaxants (NDMRs) act by competitively inhibiting the action of acetylcholine at the nicotinic acetylcholine receptors (nAChRs) on the motor endplate, leading to muscle relaxation. The resistance of a muscle to NDMRs is determined by the concentration of nAChRs and the density of acetylcholine receptors on the muscle fibers.

**Why the Correct Answer is Right**
The extraocular muscles (EOMs) are the most resistant to the action of non-depolarizing muscle relaxants. This is due to the high concentration of nAChRs on the EOMs, which makes them less susceptible to the competitive inhibition by NDMRs. The EOMs require a higher concentration of NDMRs to achieve the same level of muscle relaxation as other skeletal muscles. This is a result of the unique anatomy and physiology of the EOMs, which are essential for maintaining eye movements and alignment.

**Why Each Wrong Option is Incorrect**
**Option A:** The diaphragm is a skeletal muscle that is also resistant to NDMRs, but not to the same extent as the EOMs. The diaphragm has a high concentration of nAChRs, but the effect of NDMRs on the diaphragm is still significant.
**Option B:** The abdominal muscles are not particularly resistant to NDMRs. In fact, NDMRs can cause significant muscle relaxation in the abdominal muscles, leading to respiratory compromise.
**Option C:** The intercostal muscles are also susceptible to NDMRs and can lead to respiratory failure if not managed properly.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the EOMs are resistant to NDMRs, and a train-of-four (TOF) ratio of 0.4 or greater indicates that the EOMs are not adequately relaxed. This is a critical consideration in anesthesia and critical care medicine, as inadequate relaxation of the EOMs can lead to complications such as ophthalmoplegia.

**Correct Answer:** C. Extraocular muscles.