#### Answer

a) $2.44\times10^8$
b) $4.85\times10^{-1}$
c) $1.33\times 10^{-1}$
d) $2.86\times10^{-1}$

#### Work Step by Step

a) Retain 3 significant figures: $2.44\times10^8$
b) Retain 3 significant figures:
$4.85\times10^{-1}$
c) Retains 3 significant figures:
$1.33\times 10^{-1}$
d) $(23.56-2.3)$ must retain 1 digit after the decimal, because that's the same as the one with the fewest (2.3): $21.26\rightarrow 21.3$
Along with 1.68, this number has the fewest significant figures (3), and since all the other operators are either multiplication or division, that's the number of significant figures of the answer:
$2.86\times10^{-1}$