The correct option is A Having been a swimmer
A participle is a verb form that ends in -ing or -ed (present participles end in -ing and typically, past participles end in -ed) and functions as an adjective, as it modifies a noun. A participial phrase consists of a participle and other words, like a preposition or a noun. In the given sentence, "having" is a participle, the words "been a swimmer" that follow, make it a participial phrase, as this phrase "having been a swimmer" modifies the noun "Sameer", hence option A is correct. "Sameer knew" is a phrase made of the subject "Sameer" and the verb "knew", hence it's not a participial phrase and option B is incorrect. "Importance of swimming" is a noun phrase, made of a noun, preposition, and gerund, respectively; hence option C is incorrect. "Swimming" is a gerund here, i.e., a verb form that functions as a noun. Though gerunds and participles have the same form, ending with ing, they can be distinguished on the basis of their context in a sentence. Here, "swimming" performs as an object of the verb "knew", hence it's a gerund and option D is incorrect.