All Primary Monkeys in radius get tier 1 upgrades for free, increased range and reduced ability cooldowns.
~ BTD6 description
Primary Mentoring is the fourth upgrade of Path 1 for the Monkey Village in BTD6. It allows all Primary Monkeys within its radius to be granted free tier 1 upgrades. Starting from Version 17.0, it also grants nearby Primary Monkeys with +5 range and reduces their ability cooldowns by 10%.
It costs $2125 on Easy, $2,500 on Medium, $2700 on Hard, and $3000 on Impoppable.
For the free Tier-1 upgrades, Primary Mentoring is best used with Bomb Shooters, as their Tier-1 upgrades collectively are the most expensive of their kind.
The two most expensive Tier-1 Bomb Shooter upgrades are Bigger Bombs ($400 on Medium) and Faster Reload ($250 on Medium), saving up to $650 on Medium. As such, certain Striker Jones + Bomb Shooter strategies, particularly with either masses of MOAB Maulers or Recursive Clusters, will benefit the most from the discount portion of the Primary Mentoring upgrade. For 2-0-4 Recursive Clusters, this discount is slightly less than that for 1-3-0 MOAB Mauler spam, but each 2-0-4 Recursive Cluster can save to $600 on Medium ($400 + $200, Medium prices) per Bomb Shooter if upgraded within range of the Primary Mentoring.
Because of the tiny ranges of most Ice Monkeys and Tack Shooters, the +5 range buff is much more significant of a range increase for those tower types.
This especially empowers the Super Brittle + Tack Zone synergy, because these two towers hand-in-hand support each others' weaknesses; Super Brittle is a poor damaging tower on its own, but its massive damage debuffs against affected bloons plus its moderately adequate stalling power against Super Ceramics allow the Tack Zone to reap up large amounts of damage, especially against Super Ceramics, the bloon types that Tack Zone may typically struggle heavily against.