My brother ---- a motor bike with the money I gave him.

The correct verb tense for this sentence is: bought.

Here's why:

• Buy is the present tense, indicating an action that typically happens in general.
• Brings suggests obtaining something and moving it to a location. While it's possible your brother might bring the motorbike to you, the sentence focuses on the acquisition itself.
• Buys is present tense, indicating an ongoing action. It wouldn't be suitable for a completed action in the past.
• Bought is the past tense, perfectly capturing the fact that your brother used the money you gave him to complete the action of acquiring a motorbike in the past.

Therefore, "bought" most accurately reflects the past action of your brother using your money to purchase a motorbike.