The correct option is B friendly
The word 'forbidding' refers to being unfriendly or threatening in appearance.
Option A, lenient refers to being more merciful and tolerant than expected.
Option B, friendly refers to being kind and pleasant.
Option C, tranquil refers to being calm and free form disturbance.
Option D, mild refers to being not severe, serious or harsh.
Thus, according to the meaning, the word 'friendly' is the opposite of the word 'forbidding'.