The speed of light is now defined to be c=2.99792458x10^8 m/s. express the speed of light to three significant figures

Easy. All you need to do us to is to shorten the number so that it only contains 3 digits. Change 2.9979248 to 2.99. Don't forget to round the number to the last possible digit. In this case, 2.997 would round to 3.00

So the answer would be 3.00x10^8.

Why?

Well, every non-0 number to the left of the decimal point will count as a significant figure. And every number to the right of the decimal point, whether it's 0 or not, will also count as a significant figure.

The 3 to the left of the decimal point and the 2 0s to the right will add up to 3 significant figures.

It should be d if not then c