Which element must be present in an organic compound? (1) hydrogen (3) carbon(2) oxygen (4) nitrogen

the answer is 3) carbon

main component of organic compounds are the element carbon

C forms covalent compounds with other C atoms and form long chains resulting in forming compounds that are very useful to us in our day to day life

human body is made of 4 types of organic molecules.

C covalently bonds with other atoms such as H, O and N etc. forming a large array of useful compounds

a polar bond is formed when electrons are shared unevenly between atoms

the best answer is d because the strong cohesion is caused by the ability of water molecules to undergo hydrogen bonding.

a is incorrect because when water solidifies, it becomes   ice, which expands and occupies a bigger volume for the same mass of water in its liquid state.

b is incorrect because water has high surface tension. besides bonding to itself it hydrogen bonds to other molecules at the top decreasing possibility for breaking of attractive forces.

c is incorrect because the water molecule has oxygen which is highly electronegative and so pulls the electron pair towards itself creating a dipole moment, making the molecule polar.

e is incorrect because water has strong adhesion, ability to stick to other molecules such as glass because of this molecules having stronger hydrogen bonding than water.