The correct answer is in.
The word “in” is used to express support for or approval of something. For example, “I am in favor of the idea” means that you support the idea.
The word “on” is used to express a position or opinion on something. For example, “I am on the fence about the idea” means that you are not sure whether you support or oppose the idea.
The word “of” is used to express possession or ownership of something. For example, “I am of the opinion that the idea is a good one” means that you believe that the idea is good.
The word “about” is used to express concern or interest in something. For example, “I am about the idea” means that you are interested in the idea.
In this case, the speaker is expressing support for the idea, so the correct word to use is “in”.