**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination of the central nervous system (CNS), leading to a wide range of neurological symptoms. One of the key features of MS is its impact on ocular motility, resulting in various forms of nystagmus.

**Why the Correct Answer is Right**
Nystagmus in MS is typically associated with the abducting eye, due to the involvement of the medial longitudinal fasciculus (MLF) and the internuclear ophthalmoplegia (INO) it causes. The MLF is a critical structure that facilitates conjugate eye movements, and its damage leads to impaired adduction and nystagmus on the abducting eye. This is a hallmark feature of MS, and its presence can be a diagnostic clue.

**Why Each Wrong Option is Incorrect**
**Option A:** Nystagmus on abducting eye is a correct association with MS, as explained above. It's a key feature of the disease.
**Option C:** One and half syndrome is a condition that can occur in MS, characterized by impaired adduction and preserved abduction of one eye, along with impaired conjugate gaze to the opposite side. This is another manifestation of MLF damage in MS.
**Option D:** Pendular nystagmus is a type of nystagmus that can be seen in MS, particularly in patients with severe disease or those with lesions affecting the brainstem. It's a high-yield feature of MS.

**Clinical Pearl / High-Yield Fact**
In patients with MS, nystagmus can be a valuable diagnostic clue, particularly when associated with other features such as internuclear ophthalmoplegia or one and half syndrome. Always consider MS in the differential diagnosis of patients presenting with these ocular motility disorders.

**✓ Correct Answer: B. Nystagmus on adducting eye**