they didn,t go for a walk because it________ a)rained b)was raining

It is important to remember, that very often the choice of correct tenses depends on the situation (context).  Sometimes, only one sentence is not enough to describe this context and then, it is possible to use more than one correct tense. For example, in this sentence we can use two different tenses, but the emphasis in each case would be different. Compare:

1. They didnt go for a walk because it was raining heavily, no umbrella could protect. 

2. They didnt go for a walk because it had rained. There were huge puddles everywhere and a lot of mud. Here the rain was finished before they wanted to go but its results prevented them from going out.