Which of the following is an example of a correctly punctuated sentence?

The selected answer is an example of a correctly punctuated sentence because it properly uses a semicolon to connect two independent clauses. The phrase "I was planning to stop by the grocery store" and the phrase "I realized that my tire had gone flat" can stand alone as complete thoughts. When joining such independent clauses with a transitional phrase like "however," a semicolon is the appropriate punctuation before the transitional word, followed by a comma.

Using a semicolon indicates a close relationship between the two ideas while also maintaining their independence. The transitional word "however" introduces a contrast between the two thoughts, showing that the speaker had an intention that was thwarted by an unforeseen circumstance.

In contrast, the other choices demonstrate improper punctuation for connecting independent clauses. For instance, using a comma before "however" without the preceding semicolon creates a run-on sentence. Similarly, placing a semicolon directly before "however" without the necessary punctuation first creates a grammatical mistake. These errors contribute to the confusion and misalignment of the sentence structure. Thus, the correct choice effectively conveys a cohesive thought while adhering to the rules of punctuation.