In order to know which has the greatest mass, it would be useful to convert the choices into the base units which is grams. We do as follows:
a. 2.3 x 10^9 pg = 2.3 x10^-3 g
b. 1.5 x 10^8 np = 0.15 g
c. 6.6 x 10^5 ug = 0.66 g
d. 8.4 x 10^2 mg = 0.84 g
e. 3.0 x 10^-1 g = .30 g
Therefore, it is clear that the correct answer is option D, 0.84 g.
Hope this answers the question. Have a nice day.