Which sentence is a run-on sentence? Α.
Sarah, Ben's other daughter, was not interested in hiking so she stayed home.
B. Although Izzy was young, she had been on many hikes with her dad.
C.
Ben and Izzy both enjoyed hiking, so they went hiking together often.
D.
Ben wanted to make sure Izzy was prepared to hike; therefore, he taught her hiking safety.

The answer is None.

Explanation:

A run-on sentence occurs when two or more independent clauses (i.e complete sentences) are connected improperly. What this means is that there is no punctuation mark or conjunction in between.

In the options above, there's no run-on sentence because they are all properly punctuated and the proper conjunctions are used to join the sentences. They also have one complete and one incomplete sentence thereby making them not to be run-on sentences.

To derive a sample run-on sentence using option C would be:

Ben and Izzy both enjoyed hiking they went hiking together often.

Notice the difference between this and the original option. The absence of "so"

When "so" is removed, they become two complete sentences i.e (Ben and Izzy both enjoyed hiking. They went hiking together often.) Thereby making it a run-on sentence. But because the original option has "so", it's not a run-on sentence.

The same can be shown in other options. The presence of punctuation and proper conjunctions used in each of the options above, means there is no run-on sentence.

B.  

Ben and Izzy both enjoyed hiking, so they went hiking together often.

Explanation: