Topic: Degree of Comparison,

Best Answer Explained in Below-:

The superlative degree of an adjective is used to indicate the highest degree of a quality among three or more subjects. In the case of the word "able," it is an adjective meaning competent or capable. To form the superlative degree, adjectives typically follow certain rules based on their syllable count.

For one-syllable adjectives, such as "big," we typically add "est" to form the superlative (e.g., biggest). For two-syllable adjectives that end in "y," we also change the "y" to "i" and add "est" (e.g., happy to happiest). However, for multi-syllable adjectives such as "able," we cannot simply add "est." Instead, we use the phrase "most" to form the superlative. Therefore, the correct way to express the superlative degree of "able" is "most able."

The options provided—able, unable, and abler—do not correctly convey the superlative meaning. "Able" is the positive form, "unable" is its antonym, and "abler" refers to a comparative form that, while grammatically correct, is not commonly used. Thus, the correct answer is D) none of the above, as "most able" is the accurate superlative expression.