Which structure in plants produce a substance that protects the extending roots from developing friction?

The right answer is the root cap.

In botany, the root cap represents the end of a root that serves both to protect the apical meristem of the root, to facilitate its penetration into the soil through mucigel and to control the reaction of the root to gravity through the columella containing statocytes. The perception of gravitropism is thus located in the headdress of the root.

This root apex may be lacking in certain plants such as submerged plants (ceratophylum) or horse chestnut.

multiply it by 5 in order for you to understand i cant give an exact answer.