The correct option is
A Rigid
Antonyms are words that have contrasting, or opposite, meanings.Choice A is correct. Flexible means 'bending and not breaking' the opposite being rigid meaning 'firm'. For example: The rigid piece of plastic did not bend but it melted in the heat. Choice B means 'lavish praise on someone with the idea of furthering one's interest. Choice C means the same as flexible. choice D means 'unmindful'. Choice B, C and D are wrong.