Glib is someone who is fluent but nonsense and shallow. so both fluent and Dull make sense.
Shaheen Mushtaq from Karachi, Pakistan is saying Dull is correct answer
Siddharth Bajpai from Salem, India is saying Dull is correct answer
sajid ali dahani from Karachi, Pakistan is saying Fluent is correct answer
m4math dd from Bhubaneswar, India is saying Thin is correct answer