**Core Concept**
The question is testing the understanding of the vascular supply to the brain, specifically the areas affected by occlusion of different arteries. The internal carotid artery (ICA) provides blood supply to the anterior two-thirds of the cerebral hemisphere, including the optic nerve and the motor cortex responsible for contralateral hemiplegia.

**Why the Correct Answer is Right**
Occlusion of the internal carotid artery can lead to a range of symptoms, including contralateral hemiplegia due to involvement of the motor cortex and diminution of vision in the contralateral eye due to compression or ischemia of the optic nerve. This is because the ICA supplies the optic nerve and the ophthalmic artery, which is responsible for blood flow to the eye. The involvement of the optic nerve leads to visual field defects, including diminution of vision in the contralateral eye.

**Why Each Wrong Option is Incorrect**
**Option A:** Middle cerebral artery (MCA) occlusion typically presents with contralateral hemiplegia, but it does not typically cause diminution of vision in the contralateral eye, as the MCA does not supply the optic nerve directly.

**Option B:** Basilar artery occlusion typically presents with symptoms of brainstem dysfunction, including cranial nerve palsies, ataxia, and dysarthria, but it does not typically cause contralateral hemiplegia or diminution of vision in the contralateral eye.

**Option C:** Anterior cerebral artery (ACA) occlusion typically presents with symptoms of contralateral lower limb weakness, but it does not typically cause diminution of vision in the contralateral eye, as the ACA does not supply the optic nerve directly.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that occlusion of the internal carotid artery can lead to a range of symptoms, including contralateral hemiplegia, diminution of vision in the contralateral eye, and other symptoms such as aphasia and spatial disorientation.

**✓ Correct Answer: D. Internal carotid artery. Occlusion of the internal carotid artery can lead to contralateral hemiplegia and diminution of vision in the contralateral eye.**