What part of speech is "best" in the sentence, "It is best to make an appointment early in the day."
Unless "best" is followed by a noun which it is modifying, the word is being used as an adverb.
http://www.answers.com/best
You can see here -- and in any dictionary -- that they are telling you the part of speech for the word. In this case, they indicate that it can be either an adjective or an adverb. So ... if there's no noun right after it to be modified,* it is serving as an adverb.
=)
*An example of such a sentence would be this: She is wearing her best coat tonight. ("Best" is the superlative of "good" and it's modifying "coat.")
In the sentence "It is best to make an appointment early in the day," the word "best" is being used as an adverb. The function of an adverb is to modify a verb, adjective, or another adverb. In this case, "best" is modifying the verb phrase "to make an appointment." It is describing the manner or quality in which the action should be performed.
To determine the part of speech of a word, you can consult a dictionary. In this case, you can look up the word "best" in any dictionary, and it will indicate that it can be used as either an adjective or an adverb. In the sentence provided, since "best" is not followed by a noun that it is modifying, it is being used as an adverb.
For example: "She is wearing her best coat tonight." In this sentence, "best" is modifying the noun "coat," and it is being used as an adjective.