Which of the following is called Energy of activation or E act?

The energy of activation is the additional energy required for reactions to occur spontaneously. It is the energy required and available for a reaction to happen. Hope this answers the question.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.