The correct option is D Hostile
The word 'friendly' refers to being kind and pleasant.
Option A, helpful refers to being ready and willing to give help.
Option B, understanding refers to being sympathetically aware of other person's feelings.
Option C, quiet refers to making little or no noise.
Option D, hostile refers to showing or feeling opposition or dislike.
Thus, according to the meaning, the word 'hostile' is the opposite of the word 'friendly'.