The word "tangentially" means in a way that is slightly or indirectly related to the main topic or course of action. To find the best antonym (opposite) for "tangentially," we need to look for a word that means directly, openly, or closely related to the main topic or course of action.

Option 3: "Candidly" means openly, honestly, and directly. It is the best antonym for "tangentially."

Option 1: "Partly" means to a certain extent or partially, but it doesn't necessarily imply directness or the opposite of "tangentially."

Option 2: "Forcefully" means with force or strength, which is not directly related to the concept of being tangential.

Option 4: "Fitfully" means irregularly or intermittently and doesn't directly contrast with the idea of being tangential.

So, the best antonym for "tangentially" is Option 3: "Candidly."