All Primary Monkeys in radius get tier 1 upgrades for free.
~ BTD6 description
Primary Mentoring is the fourth upgrade of Path 1 for the Monkey Village in BTD6. It allows all Primary towers within its radius to be granted free tier 1 upgrades. It costs $2125 on Easy, $2,500 on Medium, $2700 on Hard, and $3000 on Impoppable.