Which word requires a spelling change before adding the sufix - er a fast b merry c smart d buy

The answer is letter b. merry.

Suffix is sometimes termed as postfix, an affix placed after or at the end of a stem of a word. Suffixes are considered as word endings which can change or add meaning to the word. In adding suffixes to a stem of word, there are rules to follow. One of which is the changing of - y ending of an adjective to i and add er (-er is one of the most commonly used suffix in linguistics specially to comparatives).

For example, the word merry becomes merrier.

merry = > merri + er

y is changed into i and er is added, thus makes the word merry to merrier.