Step 1: Understand the Definition.
The phrase "The act of afforestation by planting many trees" refers to the process of planting trees to create or expand forests. The term that best describes this activity is afforestation.
Step 2: Analyze Each Option.
Option (1): Deforestation — Incorrect, as deforestation refers to the removal of forests, which is the opposite of planting trees.
Option (2): Afforestation — Correct, as afforestation specifically means planting trees to create or expand forests.
Option (3): Forest — Incorrect, as "forest" is a noun referring to a large area covered with trees, not the act of planting them.
Option (4): Defrost — Incorrect, as defrosting refers to removing frost or ice, which is unrelated to planting trees.
Step 3: Final Answer.
$$
(2) \mathbf{\text{Afforestation}}
$$