This question tests the degrees of comparison in adjectives.
The sentence structure as ... as is used for the positive degree of comparison. It is used to state that two things are equal in a particular quality.
The structure is: as + [positive form of adjective] + as.
The positive form of the adjective is 'smart'.
'Smarter' is the comparative degree (used with 'than').
'More smart' is an incorrect comparative form (it should be 'smarter').
'Most smart' is an incorrect superlative form (it should be 'smartest').
Therefore, the correct word to fit into the structure as \_\_\_ as is the positive adjective 'smart'.