Answer:
The answer is complex.
Step-by-step explanation:
In order to answer this question, let us first see how many clauses does this sentence have. Sentence has as many clauses as predicates. So, predicates here are has not decided and will take place. Therefore this sentence has two clauses:
1. the club has not decided - independent clause
2. when the picnic will take place - dependent clause
When a sentence has one independent and at least one dependent clause, we call it a complex sentence.