Column I lists compounds and Column II lists the number of meso stereoisomers each has. Column I: (P) butane-2,3-diol; (Q) pentane-2,3,4-triol; (R) heptane-2,3,4,5,6-pentol, $\mathrm{CH}{}_3-(\mathrm{CHOH}){}_5-\mathrm{CH}{}_3$; (S) pentane-2,3-diol. Column II: (1) 0; (2) 1; (3) 2; (4) 4.

For a symmetric chain with n identical centres, count the left/right vectors that are palindromic and group them by $180^{\circ }$ rotation. This gives 1 meso form for n=2, 2 for n=3 and 4 for n=5 (computed by enumeration). Pentane-2,3-diol is unsymmetrical and has none. Thus P-1 meso, Q-2, R-4, S-0, which is P-2, Q-3, R-4, S-1 in Column II labels.