Entropy is really just a count of how many ways the particles in a system can be arranged while giving the same overall state, scaled through Boltzmann's relation \(S = k_B \ln W\), where \(W\) is the number of possible microscopic arrangements. A perfect crystal has every atom locked into one single, exact position in a repeating lattice, with no defects, no mixing of isotopes, and no disorder of any kind. Once we cool this crystal all the way down to absolute zero, all atomic vibration and thermal motion stops completely, so there is only one possible microscopic arrangement left, meaning \(W = 1\). Plugging this into Boltzmann's relation gives \(S = k_B \ln(1) = 0\), since the logarithm of 1 is zero. So a perfectly ordered crystal at 0 K has no disorder left to count, and its specific entropy must be exactly zero, not some small positive or negative number, which is option 4.