The first problem here is that you have two sentences when it should be one. In the second part, it would be better to use "it" rather than "that". With those corrections, the last one is neatest: 3.
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GPYGPY: If something is optional, it means that you can either do it or not do it.GPY, Please compare the following two sentences. Which one do you prefer?
sunny123GPY, Please compare the following two sentences. Which one do you prefer?1. If something is optional, it means that you can either do it or not do it.2. If something is optional, it means that you can either do or not do it.?(1) is better in my opinion. (2) is tolerable.