Which pair in the options will complete the following series?
S, T, R, U, ______, _____

The series consists of two alternating tracks:
1. Odd positions: $\text{S}(19),\text{R}(18),\dots$ (pattern is subtracting $-1$)

$$\text{Next term (5th item)}=18-1=17\to \text{Q}$$

2. Even positions: $\text{T}(20),\text{U}(21),\dots$ (pattern is adding $+1$)

$$\text{Next term (6th item)}=21+1=22\to \text{V}$$

Thus, the terms completing the series are Q, V.