Topic: Gerunds & Participles,

Best Answer Explained in Below-:

The phrase "Having gained truth, keep truth" employs a perfect participle. A perfect participle is formed by using "having" followed by the past participle of a verb—in this case, "gained," which is the past participle of "gain." This structure indicates that the action of gaining truth occurred before the action of keeping truth.

In English, perfect participles are used to convey a relationship of time or sequence between two actions. Here, "having gained truth" signifies that an understanding or realization has been achieved prior to the directive "keep truth."

The use of a perfect participle also emphasizes the completion of the first action (gaining truth) as a prerequisite for the second action (keeping truth) to take place effectively. Thus, "having gained" not only highlights the action that has already been completed but also sets the stage for the subsequent instruction, reinforcing the importance of retaining what has been obtained. This nuanced usage is what makes option B, perfect participle, the correct choice.