The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element.
Answer: The correct answer is option B.
Explanation: Reactivity of elements is defined as the tendency to loose or gain electrons.
These reactions are a type of single displacement reactions. A single displacement reaction is a type of reaction in which an element displaces another element in a chemical reaction. These are studied with the help of reactivity series.
The element which lies above in the reactivity series can easily displace the element which lies below in the reactivity series.
Option A: This reaction will not yield a stable product because Zinc lies below Aluminium in the reactivity series.

Option B: This reaction will yield a stable product because Sodium lies above Calcium in the reactivity series.

Option C: This reaction will not yield a stable product because Hydrogen lies below Magnesium in the reactivity series.

Option D: This reaction will not yield a stable product because Calcium lies below Barium in the reactivity series.

Option E: This reaction will not yield a stable product because barium lies below Lithium in the reactivity series.

Sun isn't actually planet but star. Really big ball made of gases. Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%). Sun is the biggest object in our Solar System. It's 99.86% of the total mass of the Solar System.
... wikipedia is really grat if you have questions like that ;)