To complete the sentences with the correct adjectives from the given list, we need to consider the context of each sentence.
(a) The fastest girl won the race.
In this sentence, 'fastest' is used because it describes the girl who was the quickest in the race.
(b) Pick up the shortest pencil from the bench.
For this sentence, 'shortest' is the most appropriate adjective. It describes picking up a pencil based on its length compared to others on the bench.
Adjectives are words that describe or modify nouns. In these sentences, they are being used to provide more information about the girl and the pencil, which helps in identifying specific qualities like speed or length.