Answer:
D. The positive form of the word faster is fast.
Step-by-step explanation:
The positive form of an adjective is used with as... as to indicate that there is no contrast between the two people or things being compared. In that respect, both elements have the same characteristic described by the adjective. For example: Mike runs as fast as Tim.
The rest of the options are incorrect because in statement A it is not clear whether the comparison is between two (comparative) or three or more people (superlative); in alternative B the comparative of sad is sadder; and in option C the comparative form compares two things.