Underlined carbon is sp3 hybridised in (a) CH CH CH (b) CH CH NH (c) CH CONH (d) CH CH CN

If carbon is sp3 hybridized. All four bonds of the carbon should be sigma bond. There should not any pi bond. In the case of sp2 hybridized at least one bond of the carbon should be a pi bond. And in the case of sp hybridized at least two bonds of the carbon should be a pi bond. Now in ethylamine, the marked carbon is sp3 as that carbon has all sigma bonds.