A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.FU, LO, RI, ?

Each term consists of opposite letter pairs (whose positional sum is $27$). Let's look at the first letters of each pair:$\text{F}(6)\stackrel{+6}{\to }\text{L}(12)$$\text{L}(12)\stackrel{+6}{\to }\text{R}(18)$$\text{R}(18)\stackrel{+6}{\to }\text{X}(24)$ The opposite letter of $\text{X}$ is $\text{C}$ (since $24+3=27$). Therefore, the next term in the series is XC.