When 2-methyl-1,3-butadiene (isoprene) undergoes a 1,4 addition of hydrogen chloride, the major product that is formed is 1-chloro-3-methyl-2-butene. Little or no 1-chloro-2-methyl-2-butene is formed. How can you explain this

The rearrangement of the intermediate molecule, i.e,  the double bond shifted to the 2nd and the 3rd carbon, allows the addition of hydrogen to the 1st carbon and the addition of chloride to the 3rd carbon

Explanation:

The major product formed is always dependent on the fact that how much stable is the intermediate molecule. Isoprene is a conjugated diene and when it reacts with hydrogen chloride the double bast at the 1st carbon attacks the hydrogen of HCl leaving the 2nd carbon with a positive charge  (carbocation formed between carbons having conjugation). The double bond from the 3rd carbon then shifts to the 2nd carbon for stabilization which ensue the addition of chloride to the 4th carbon. Now the nomenclature of the product according to IUPAC would become 1-chloro-3-methyl-2-butene.

Here it should be noted that when an electrophilic addition reaction occurs in conjugated dienes system, the reagent attach itself to the terminals of the system rather than the terminals of the double bond (as observed in this case).

a is the correct answer