Answer:
The answer that is the most effective substitute for the underlined part of the sentence is "join the Science Club, you may sign up"
Explanation:
The example sentence is an If clause, or an if conditional, this kind of sentences connect the two ideas with a comma when the first sentence starts with the word "If", the punctuation of the sentence is incorrect by using a semicolon instead of a comma to connect the two part of the complete clause into a single one.
