Explanation

(b)
2-pentanone and 3-methyl-2-butanone are chain isomers because they differ in carbon skeleton.
$CH_{3}CH_{2}CH_{2}COCH_{3}\ \ \ \ \ \ \ \ \ \ \ CH_{3}CHCOCH_{3}$
2-pentanone |
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
3-methyl-2-butanone