Present simple/Present continuous I brush my teeth now. am brushing is brushing

The correct form to use in this context is the present continuous: "I am brushing my teeth now."

Here's a brief explanation of both tenses:

• Present Simple (I brush): This tense is used for actions that are habitual or generally true. For example: "I brush my teeth every morning."

• Present Continuous (I am brushing): This tense is used for actions that are happening at the moment of speaking. For example: "I am brushing my teeth now."

So in your sentence, since you are talking about the action currently happening, "I am brushing my teeth now" is the appropriate choice.