Answer:
Smartest
Explanation:
When we want to use an adjective in order to make a comparison between two things, we can either use the comparative or the superlative form. The comparative employs the suffix "-er." Some examples of this would be <em>cleaner, louder </em>or <em>smarter</em>. When the adjective is particularly long, this is substituted by the word "more." Some examples are <em>more powerful, more intelligent, more capable</em>.
On the other hand, the superlative is used to indicate that the object we are talking about has the highest level of such adjective. For this, we would use the suffix "-est" (<em>cleanest, loudest, smartest</em>) or the word "most" (<em>most powerful, most intelligent, most capable</em>).