Express the sum of 9.78 g and 7.0 g using the correct number of significant
digits.

C. 16. 8 g

Explanation:  

when we are using significant numbers we have to keep in mind if the last digit is over 5 or down 5 to increase or decrease 1 digit the number to the left, if the last number is down 5, we decrease the number to the left 1 digit, but if the number is over 5, we can increase 1 digit to the number to left.

then we have this equation, as the 0 is not a significant number, we can omit this one.

9.78 g + 7.0 g = 16.78 g

as the last number is over 5 we can increase 1 digit to the number to the left, the we get the following result.

16.78 g = 16.8 g

because of that the correct answer is C. 16. 8 g

a:

by the combination of an unlimited number of elements