Find the synonym of the underlined word.
Rachel thinks she is overweight, so she will go on a crash diet.

The correct option is C Obese

'Overweight' means above a weight considered normal or desirable.

"she was a stone overweight"

In this context, "Rachel thinks she is overweight, so she will go on a crash diet", 'overweight' means obese. Hence, option B is correct and the other options are not because they do not provide the appropriate information.