Among the following an empty set is

By options $P=\{2,4,6,8,\cdots \}\to$ infinite

$Q=\{\text{duck, penguin, }\cdots \}\to$ not an empty set

$R=\{ \}$, as no real number is both integer as well as irrational $\to$ Empty

$S=\{a,b,c,d,\cdots z\}\to$ Finite, not an empty set.