Answer
3.00 $\times 10^{8}$ m/s
Work Step by Step
We know that if we want to round off a number and it is greater than $5$, then the remaining digit to the left will increase by $1$. Thus, the speed of light of $2.9979\times 10^8m/s$ in three significant digits is given as:
$c=3.00\times 10^8m/s$
