23. The building blocks of elements are called:
a) Atoms

Because except for a handful of gases that can only exist as molecules (Oxygen, Hydrogen, Nitrogen, Chlorine) , all other elements are composed of atoms. Molecules are building blocks of compounds not elements.

27) If an alkali is slowly added to an acidic solution, the pH of the acidic solution will:
d) Decrease to 7 and then increase

A is not right because adding alkali will lower pH of acidic solution.

31) Information can be sent over long distances in the form of:
Can you explain this, I am sceptical.

10. When a ray of sunlight enters a dark room , its straight path become visible because of dust particles hanging in the air. It is because light is :
Light itself is invisible. You can see things clearly in presence of light, can't you? Had light been visible our view would have been murky at least :P
b)Transparent