The correct option is A smaller
Comparative adjectives are used to compare differences between the two objects they modify (larger, smaller, faster, higher). They are used in sentences where two nouns are compared, in this pattern: Noun (subject) + verb +comparative adjective + than + noun (object). Usually in words with one syllable, the comparative degree is obtained by adding an -er to the positive degree. Similarly in this question, "small" (Option D) is the positive degree. To make it to a comparative one, we will add an -er to it, which makes it "smaller". Thus Option A is correct. Option B and C gives the superlative degree of the adjectives.