I just did that with my 8 year old this morning.

Draw a line from 0 to 500 and divide each 100 into 2 or more sections.
Ask her to approximate where her number is and tell her to round up (or down) to the nearest 100. Then ask her to add the two numbers.
It's not a calculating problem, only estimating.

BTW, the answer is A and C.
B only gets you to 200