Answer: 70
Reason:
The left-most digit is the most significant when it comes to rounding. We bump the 6 up to 7 since 68 is closer to 70 (compared to 60). The other digits turn to 0 and become insignificant.
Do not write any of the following:
because that would mean we would have 2, 3, and 4 sig figs respectively. We simply write 70 without a decimal point.