Please solve this question :

Dear Student, maximum torsional strain occurs when the torsion or dihedral angle between larger groups (that can cause steric hindrance) is minimum i.e. 0°. In other words, the fully eclipsed conformation has maximum torsional and Vanderwaal strain. So, correct answer is (b). Option d also has torsion angle of 0° but it is not the confirmation for isopentane as it has total of 6 carbon atoms. Regards.