The correct answer is the first option.
The sentence that presents the correct subject-verb agreement is the first one since in it we are talking about "heavy rains" OR "fast-melting snow". In other words, the subject is coordinated by the conjunction "OR" which makes the subject singular. The verb "causes" is conjugated in the third person singular.
The outcome of Macbeth's conflict with Banquo and Fleance is that Macbeth tries to have them both murdered, but Fleance escapes. In Act III, Scene iv, as the murderers are trying to complete their task, one says "There's but one down, the son is fled". This shows that Banquo has been successfully killed but Fleance has escaped.
