Final answer:
The correct answer is 'B) started'. This is because in English, the past continuous tense is used for longer actions and the past simple tense for shorter, completed actions happening during that longer action.
Step-by-step explanation:
The correct option for this English grammar question is 'B) started'. In English, when we want to express that one action happened during another longer action, we use the past continuous tense for the longer action and the past simple tense for the shorter, completed action. Hence, the 'boys were playing games' (longer action - past continuous tense) when 'it started raining' (shorter, completed action - past simple tense). This sentence implies that the boys had already begun playing games before the rain started.
Learn more about Tenses in English