Final answer:
The sentence should be completed with 'get permission,' resulting in 'If we get permission, we will play basketball at school.' This use of the simple present tense in the if clause conveys a possible future action.
Step-by-step explanation:
The correct way to complete the sentence "If we (get) .......permission, we will play basketball at school." is by using the simple present form of the verb 'get,' which results in the sentence: "If we get permission, we will play basketball at school." In English, the simple present tense is used in the 'if' clause of conditional sentences to talk about something that might happen in the future. Remember that having permission to do activities like playing basketball at school is important because it ensures that such activities are organized with the school's knowledge and consent for student safety and coordination.