Final answer:
The sentence 'The plants are turning brown; we need to water them more often' is a complete sentence because it contains two independent clauses that are joined with a semicolon, which is properly utilized to connect closely related complete thoughts.
Step-by-step explanation:
You asked if the sentence 'The plants are turning brown; we need to water them more often' is a complete sentence, a sentence fragment, or a run-on sentence. The sentence you have provided is a complete sentence, option C. It consists of two independent clauses 'The plants are turning brown' and 'we need to water them more often.' These two clauses are joined correctly with a semicolon, signifying a close relationship between the two ideas. A semicolon is correctly used here because both parts of the sentence are complete thoughts that could stand alone as separate sentences, but are closely related to each other. This is not a sentence fragment because both clauses have a subject and a verb, and express a complete thought. It is also not a run-on sentence because the clauses are joined with proper punctuation.