Answer:
The sentence that shows the correct sentence construction is B.
Step-by-step explanation:
"Sitting by the seaside" is a participle phrase. A participle is a form of the verb that functions as an adjective by modifying nouns and pronouns. A participial phrase includes the participle and its complements and modifiers.
In this case, I believe that B is the right answer because the participial phrase is followed by the element it modifies. The one sitting by the seaside is the speaker (I). A lacks a comma to be punctuated correctly, in C there is a dangling modifier because the participial phrase has nothing to modify or describe (the NP "the serenity" cannot sit by the seaside since it has no volition), and D is wrongly punctuated.