For which of the following do you need to double the final consonant before adding the suffix?

cost + - ly

shop + - er

limit + - ed

fear + - ing

Among the following words, the choice that you need to double the final consonant before adding the suffix to the base word is, shop + - er. The word shop plus - er becomes shopper with the double consonant p. The rule applied here is that, if the word is only one syllable and ends in a vowel and consonant, double the last letter before the vowel suffix.