C. Right anterior cerebral artery: The right anterior cerebral artery supplies the medial aspect of the right hemisphere, which controls the left leg. Weakness in the left leg (2/5 power) with normal strength in the right leg and all upper limbs suggests a lesion in the right hemisphere, specifically affecting the area supplied by the anterior cerebral artery.

B. Left anterior cerebral artery: Although less likely given the typical vascular anatomy and the presentation, it's conceivable that an unusual vascular variation or a specific lesion could affect the left anterior cerebral artery and cause similar symptoms. However, this would be less common.
A. Right Middle cerebral artery: The middle cerebral artery primarily supplies the lateral aspect of the hemisphere, which is more associated with arm and face function rather than leg function. While it's possible for a large middle cerebral artery stroke to affect leg function, especially if the stroke is extensive, the primary presentation of weakness in one leg would be less typical for this artery.

D. Mid basilar artery: Although a stroke in the mid basilar artery would more commonly present with brainstem signs, cerebellar signs, or bilateral long tract signs, it's crucial not to miss this diagnosis due to its potential for severe and rapid deterioration, including locked-in syndrome or death. The presentation could be atypical, especially in the early stages.

Unusual vascular anomalies or Supply from an unusual source: Rarely, the blood supply to the brain can be anomalous, leading to unexpected patterns of weakness. These would be rare and usually identified through detailed vascular imaging.
Other rare causes of stroke: Such as vasculitis, moyamoya disease, or other non-atherosclerotic vasculopathies, which could present with unusual patterns of weakness but would be identified through a combination of clinical presentation, imaging, and laboratory tests.