Let's analyze the problem step-by-step.
We want to determine which word requires a spelling change before adding the suffix "-er."
The rule in question states that if a word ends in "y" and the letter before "y" is a consonant, then the "y" changes to "i" before adding the suffix "-er."
Let's check each word:
a. fast
- The word "fast" does not end in "y," so no change is needed.
b. merry
- The word "merry" ends in "y," and the letter before "y" is "r," which is a consonant.
- According to the rule, "merry" would change to "merrier."
c. smart
- The word "smart" does not end in "y," so no change is needed.
d. buy
- The word "buy" ends in "y," but the letter before "y" is "u," which is a vowel.
- Therefore, it does not require a change; it would just become "buyer."
Based on the analysis above, the word that requires a spelling change before adding the suffix "-er" is:
b. merry
Therefore, the answer is:
b. merry
