Which of the following treatments softens the wood, making it easier to climb?

Ammoniacal copper fatty acid treatments are known for their ability to soften wood, which can make the wood more pliable and easier to work with, particularly in scenarios requiring climbing or grip. This treatment works by penetrating the wood fibers, altering their structure in a way that facilitates easier manipulation without compromising the integrity of the wood significantly. This characteristic is especially useful in applications where enhanced usability of the wood is desired.

In contrast, other treatments such as pentachlorophenol, chromate copper arsenates, and copper naphthenate are primarily focused on wood preservation and resistance against decay and insect infestation rather than on altering the wood's density or easiness to climb. These treatments are designed to provide long-term stability and protection, making them less suitable when the goal is to soften the wood for a specific functional purpose like climbing.