Topic: Interchange of Parts of Speech,

Best Answer Explained in Below-:

The question asks for the verb form related to the noun "strength." Understanding the relationship between these words is key. "Strength" refers to the quality or state of being strong, indicative of power or force. To express the action of making something stronger or increasing its strength, we use the verb "strengthen."

Let's analyze the options:

- A) **strengthen**: This is the base verb form that directly conveys the action of making something stronger. For example, you can say, "Exercise can strengthen your muscles."

- B) **strong**: This is an adjective that describes someone or something that possesses strength. It does not convey an action, hence not fitting the criterion of a verb.

- C) **stronger**: This is the comparative form of the adjective "strong," used to compare the strength of two subjects. It does not indicate an action and is therefore not a verb.

- D) **strengthening**: While this is the present participle form of the verb, it still does not represent the base action verb itself.

Thus, the correct answer is A) strengthen, as it is the verb directly derived from the noun "strength," representing the action of enhancing or increasing strength.