Choose the word that best replaces the words highlighted in the sentence.…

A word created by joining together the initial letters of a longer name or phrase, and then used as an ordinary standalone word, is classed as an abbreviation — specifically, an acronym, the type of abbreviation that is pronounced as a single word rather than spelled out letter by letter.

RADAR is built exactly this way: it takes the initial letters of RAdio Detection And Ranging and is read as one word. So the phrase “a word formed from the initial letters of a set of words” is best replaced by “an abbreviation.”

• an anachronism describes something placed in the wrong time period (for example, a clock shown in a medieval painting) — it has nothing to do with how a word is built from letters.

• an anagram rearranges every letter of one word or phrase to spell a different word (for example, 'listen' becomes 'silent') — RADAR is not a rearrangement of another word's letters, it is built from the first letters of several separate words.

• an idiom is a fixed multi-word expression with a figurative meaning (for example, 'kick the bucket') — it stays a phrase, not a single word built from initials.