This is how I finally understood why B is sufficient.
The argument looks like this if we put assumtion B, which is a hidden premise, as a premise in the argument.
Prem 1: Forest fires are natural phenomenon
Prem 2: Forest fires are required for many forests to flourish
Prem 3: (B) Protection of forest is the only legit reason to prevent or control forest fire.
=> But because forest fires are required for many forests to flourish, preventing or controlling the forest fire can't be the legit reason since it doesn't protect the forest.
Conc: (Because it does not have any legit reasons) Systematic attempts by human beings to prevent or control forest are ill-advised and shortsighted.
This is how I finally understood why B is sufficient.
The argument looks like this if we put assumtion B, which is a hidden premise, as a premise in the argument.
Prem 1: Forest fires are natural phenomenon
Prem 2: Forest fires are required for many forests to flourish
Prem 3: (B) Protection of forest is the only legit reason to prevent or control forest fire.
=> But because forest fires are required for many forests to flourish, preventing or controlling the forest fire can't be the legit reason since it doesn't protect the forest.
Conc: (Because it does not have any legit reasons) Systematic attempts by human beings to prevent or control forest are ill-advised and shortsighted.