The correct option is C Friendly
The words 'intimidating' refers to having a frightening, overawing or threatening effect.
Option A, authoritative refers to being commanding and self-confident.
Option B, casual refers to being relaxed and unconcerned.
Option C, non-serious refers to being not significant or dangerous.
Option D, friendly refers to being kind and pleasant.
Thus, according to the meaning, the word 'friendly' is the farthest in meaning from the word 'intimidating'.