The correct option is B hostile
The word 'friendly' refers to being kind and pleasant,
Option A, hostile refers to being unfriendly and showing dislike.
Option B, quiet refers to making little or no noise.
Option C, understanding refers to having sympathetic awareness and tolerance.
Option D, helpful refers to giving or ready to help.
Thus, according to the meaning, the word 'hostile' is the opposite of the word 'friendly'.