Explanation

(a)
As the molecular mass increase, boiling point increases. In case of isometric amines, however, as the number of H-atoms attached directly to N-atom decreases, boiling point decreases because tendency to form H-bonds decreases. Hence, the order of boiling points of given amines is
$CH_{3}NH_{2} < \left( CH_{3} \right)_{3}\ N < {CH}_{3}CH_{2}N\left( NH_{3} \right) < CH_{3}CH_{2}CH_{2}NH_{2}$