Answer
(a) Organic compounds are made up of C-C bonds which are covalent in nature. Such molecules are held together by weak London dispersion forces which are easy to be broken by supplying less energy. Thus, they have low melting and boiling points.
(b) Sugar when heated doesn't melt, it rather decomposes. So, it has no boiling point.
Work Step by Step
(a) Unlike ionic compounds which are held together by strong electrostatic attraction, organic compounds have weak London forces acting between them. Hence, they are easy to be separated with less energy. So, they have low melting and boiling point.
(b) Sugar has a melting point of 186 degree Celsius. It hardly melts at such high temperature and decomposes-this is called as caramelization. As, it doesn't melt, it cannot boil upon further heating. So, it has no boiling point.
