**Explanation:**

To determine the acidity of the given compounds, we need to consider the stability of the corresponding conjugate bases. The more stable the conjugate base, the stronger the acid.

1. **Phenol (A):**
Phenol has a hydroxyl group (-OH) attached to a benzene ring. The lone pair of electrons on the oxygen atom can delocalize into the benzene ring, resulting in resonance stabilization. This resonance stabilization increases the stability of the phenoxide ion (conjugate base), making phenol a weak acid.

2. **o-Cresol (B):**
o-Cresol, also known as ortho-cresol, has a methyl group (-CH3) attached to the benzene ring at the ortho position relative to the hydroxyl group. The presence of the methyl group reduces the resonance stabilization compared to phenol. As a result, the conjugate base of o-cresol is less stable than that of phenol, making o-cresol a slightly stronger acid than phenol.

3. **p-Nitrophenol (C):**
p-Nitrophenol has a nitro group (-NO2) attached to the benzene ring at the para position relative to the hydroxyl group. The electron-withdrawing nature of the nitro group decreases the electron density on the oxygen atom, reducing the resonance stabilization compared to phenol. Therefore, p-nitrophenol is a stronger acid than phenol and o-cresol.

4. **p-Chlorophenol (D):**
p-Chlorophenol has a chlorine atom (-Cl) attached to the benzene ring at the para position relative to the hydroxyl group. The chlorine atom is electron-withdrawing, similar to the nitro group in p-nitrophenol. Therefore, p-chlorophenol is also a stronger acid than phenol and o-cresol.

Based on the above analyses, the least acidic compound among the given options is **o-cresol (B)**. o-Cresol has the least electron-withdrawing substituents and the most favorable resonance stabilization, making it the weakest acid among the given compounds.