Select the correct comparative form of the adjective: "She is _______ than her brother."

The comparative form of an adjective is used to compare two things or people. In this case, the sentence compares "she" to "her brother" regarding the trait of being smart.

The correct comparative form of "smart" is "smarter." This follows the typical pattern for one-syllable adjectives, which often take the suffix "-er" to indicate comparison. Thus, "smart" becomes "smarter" to emphasize that one person has a higher degree of that quality than another.

In contrast, the other options do not fit the rules for creating a comparative form for the adjective "smart." "Smartest" is the superlative form, used for comparing more than two subjects. "More smart" and "most smart" are incorrect because "smart" is a one-syllable adjective that does not require "more" or "most" to form its comparative or superlative. Hence, "smarter" is the appropriate choice for this comparison.