A polar covalent bond is a bond between two atoms that has a low electronegativity difference (usually 0.1 < x < 1.9, which is kind of the border between nonpolar covalent and ionic, respectively). The most prominent example of a polar covalent bond is water, where the oxygen atom has an electronegativity of 3.5 and the hydrogen atoms have electronegativities of 2.1. The electronegativity difference is 3.5-2.1 = 1.4, which is still in covalent territory, but still a polar bond. This is why lowercase delta's appear beside oxygen atoms in drawings.
Examining, the four options, we immediately see that option D is Cl-Cl. A bond between two of the same atoms inevitably implies an electronegativity difference of zero, which means a nonpolar covalent bond: electrons are equally shared. Anywhere you see two identical atoms bonding (except for metals), you will have a nonpolar covalent bond.
Equilibrium means the state of reaction which there is no net changes occur it means the rate of forward reaction and rate of backward reaction become same and also it does not mean that the concentration of both side i.e. reactant and product become equal. after gaining equlibrium position the concentration of reactant and product remain same and also chemical reaction does not stop and it continues.
