Degree of unsaturation counts the combined number of rings and pi bonds in a molecule and is found from the formula (2C + 2 - H)/2 for a compound containing only carbon and hydrogen. Substituting C = 6 and H = 10 gives (2*6 + 2 - 10)/2 = (12 + 2 - 10)/2 = 4/2 = 2. Thus the molecule contains a total of two units of unsaturation, which could be two rings, two double bonds, one ring and one double bond, or one triple bond. The answer 1 is wrong because it underestimates the hydrogen deficiency relative to the saturated formula C_6H_14. The answer 3 overestimates the deficiency and would require formula C_6H_8. The answer 0 would describe a fully saturated alkane such as hexane, which is C_6H_14, not C_6H_10. This formula-based reasoning is a standard NCERT structural tool. Sanity check: the saturated reference C_6H_14 has four more hydrogens than C_6H_10, and each missing pair equals one degree, confirming two.