Which of he following words is not a noun ?

The correct answer is B. Quench is a verb, not a noun. It means to satisfy thirst or to put out a fire, as in "quench your thirst" or "quench the flames."

The other three words all work as nouns:

• Mention is a noun meaning a reference, as in "a brief mention in the report." It is also a verb.
• Trench is a noun for a long, narrow ditch, as in "trench warfare" in World War I. It is also a verb.
• Tension is a noun for strain or stress, as in "racial tension" or "surface tension" in physics. Its -tion ending is a typical noun marker.

The core concept here is word class: a word is identified by its function in a sentence, not just its shape. Many words serve as both nouns and verbs, so the test is whether the word can head a noun phrase in ordinary use. This skill helps directly in precis writing and comprehension, where you must read grammatical roles accurately.

Trap: Aspirants often pick Trench because of its "-ench" pattern or assume every common word has a noun use. Check whether the noun sense is standard before you rule a word in or out.