Question

Which of the following is a run-on sentence?
Marta and Brent carried in a cord of firewood because it was supposed to snow for four days.
Marta and Brent carried in a cord of firewood it was supposed to snow for four days.

Answer 1

The second one. The first one joins the two clauses using the word "because."

Answer 2

Marta and Brent carried in a cord of firewood it was supposed to snow for four days.
The second sentence. 
There should be a period seperating the two clauses, Marta and Brent carried in a cord of firewood and it was supposed to snow for four days. 
Hope this helps.