Final answer:
Unguent, a healing ointment, has its opposite in 'irritant,' which causes discomfort and inflammation, contrary to the soothing nature of an unguent.
Step-by-step explanation:
The word unguent typically refers to a soothing or healing ointment, often used to moisturize the skin or aid in the healing of minor wounds or burns. In considering the options provided for its opposite, the most fitting answer would be (a) irritant. An irritant is a substance that causes slight inflammation or other discomfort to the body. This is opposed to the purpose of an unguent, which is meant to soothe and heal.