Sachin did not play so . . . . . . . as Saurav did.

Let's break down the sentence: "Sachin did not play so . . . . . . . as Saurav did."

1. The word "play" is a verb (an action word).
2. We need a word that describes how Sachin performed the action of "playing". Words that describe verbs are called adverbs.
3. Words that describe nouns (people, places, things) are called adjectives.

Now let's look at the options:

Option A: well
"Well" is an adverb. Adverbs describe verbs (actions). Since "play" is an action, "well" correctly describes how Sachin played. The structure "so...as" uses the basic form of the adverb here. This is the correct answer.
Example: He sings well. Sachin did not play so well as Saurav did.

Option B: good
"Good" is an adjective. Adjectives describe nouns (e.g., a good player, a good book). You cannot use "good" to describe an action like "play". You wouldn't say "He plays good." So, this is incorrect.

Option C: better
"Better" is a comparative form. It's used to compare two things (e.g., "Sachin played better than Saurav"). However, the structure "not so...as" typically uses the base form of the adverb or adjective, not the comparative form. So, this is incorrect for this specific structure.

Option D: best
"Best" is a superlative form. It's used when comparing three or more things, or to say something is at the highest level (e.g., "Sachin is the best player"). It does not fit this comparison between two people using "so...as". So, this is incorrect.

Therefore, the correct word to describe how Sachin "played" is the adverb "well".

The final sentence: Sachin did not play so well as Saurav did.