A. Convert the mass of the Salt weighed out to fg.

6.564×10¹⁶ fg.

Explanation:

The following data were obtained from the question:

Mass of beaker = 76.9 g

Mass of beaker + salt = 142.54 g

Mass of salt in fg =?

Next, we shall determine the mass of the salt in grams (g). This can be obtained as follow:

Mass of beaker = 76.9 g

Mass of beaker + salt = 142.54 g

Mass of salt =?

Mass of salt = (Mass of beaker + salt) – (Mass of beaker)

Mass of salt = 142.54 – 76.9

Mass of salt = 65.64 g

Finally, we shall convert 65.64 g to femtograms (fg) as illustrated below:

Recall:

1 g = 1×10¹⁵ fg

Therefore,

65.64 g = 65.64 g × 1×10¹⁵ fg / 1g

65.64 g = 6.564×10¹⁶ fg

Therefore, the mass of the salt is 6.564×10¹⁶ fg.

the answer would be c