I was surprised at his stiff attitude.

Answer: Option B

stiff : (of a person or their manner) not relaxed or friendly.
courteous : polite, respectful, or considerate in manner.
flexible : ready and able to change so as to adapt to different circumstances.
soft : easy to mould, cut, compress, or fold; not hard or firm to the touch.
lively : full of life and energy; active and outgoing.


Antonym of stiff is flexible