The correct answer is D. To make myself feel better, I decided to go for a run around the lake.
Step-by-step explanation:
The comma (,) is one of the most widely use punctuation marks in English, this punctuation mark mainly introduces short pauses in sentences and separate elements, but besides this, there are precise rules that determine in which cases comma should be used. This includes using commas to separate elements in a list; before some conjunctions such as and, but or yet; to separate elements in dates and to separate a dependent clause (clause that depends on other clause) from an independent clause (clause that does not depend on other clause and expresses a complete idea), especially if the sentence begins with the dependent clause.
In this case, "To make myself feel better" that is the first clause in the sentence is a dependent clause as it does not express a complete idea and needs a independent clause and " decided to go for a run around the lake" is an independent clause that expresses a complete idea and completes the meaning of the first clause. This implies, this sentence presents a dependent clause followed by an independent clause and because of this both clauses should be separated by a comma to be grammatically correct as both are related, but a pause is required between them. In this way, it can be concluded the sentence that uses correctly the comma is the sentence D, as a comma is being used to separate a dependent clause from an independent clause which is a correct use of comma according to grammar.