Answer:
4). Our art teacher used to let us paint only with watercolors, but now she lets us use oil paints.
Explanation:
A run-on sentence is characterized as the sentence that connects two independent sentences conveying complete thoughts inappropriately without employing an appropriate punctuation mark or coordinating conjunction. The given sentence misses a punctuation mark prior to the coordinating conjunction 'but' which makes the meaning unclear. In order to rectify this error, a comma(,) can be placed immediately before the coordinating to separate the first clause and clarify the meaning. Thus, option 4 is the correct answer.
