**Core Concept**
A suprasellar extension of a pituitary tumor can cause visual field defects by compressing or infiltrating the optic chiasm, leading to a characteristic bitemporal hemianopia. This occurs due to the tumor's proximity to the optic nerves and chiasm.

**Why the Correct Answer is Right**
The optic chiasm is located near the pituitary gland, and a suprasellar extension of a pituitary tumor can compress or infiltrate the optic chiasm. This compression leads to a bitemporal hemianopia, where patients have difficulty seeing the outer half of each visual field. The optic nerves are affected bilaterally, resulting in a characteristic "V" or "pie-in-the-sky" visual field defect. The compression of the optic chiasm can be due to the direct extension of the tumor or due to the formation of a cyst or an intrasellar mass effect.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as a unitemporal hemianopia is more commonly associated with a unilateral optic nerve lesion or a lesion affecting the optic tract.
* **Option B:** This option is incorrect as a homonymous hemianopia is more commonly associated with a lesion affecting the optic tract or the visual cortex.
* **Option C:** This option is incorrect as a peripheral visual field defect is more commonly associated with a retinal disease or a peripheral retinal detachment.

**Clinical Pearl / High-Yield Fact**
A bitemporal hemianopia is a classic sign of a pituitary tumor with suprasellar extension, and it is essential to include this in the differential diagnosis of patients presenting with visual field defects.

**Correct Answer: B. Bitemporal hemianopia**