Fill in the blank with the most appropriate word. I __________ my teeth every morning.

The correct answer is: I brush my teeth every morning.

The other options are incorrect because:

• Brushing is the action of cleaning something with a brush. In this case, the speaker is cleaning their teeth with a toothbrush.
• Brush is a noun that refers to a tool used for cleaning something. In this case, the speaker is using a toothbrush to clean their teeth.
• Brushes is the plural form of the noun brush. In this case, the speaker is not cleaning multiple teeth, so they would not use the plural form.
• To brush is the infinitive form of the verb brush. In this case, the speaker is not talking about the act of brushing in general, but about the specific act of brushing their teeth.

Therefore, the correct answer is: I brush my teeth every morning.