Answer
$\dfrac{1}{2}$
Work Step by Step
In a roll of a die, there are $6$ possible outcomes. Since there are $3$ events corresponding to rolling a number less than 4 (a roll of $1,$ $2,$ or $3$), then the probability of rolling a number less than $4$ is
\begin{align*}\require{cancel}
P(\text{less than $4$})&=
\dfrac{3}{6}
\\\\&=
\dfrac{\cancel3^1}{\cancel6^2}
\\\\&=
\dfrac{1}{2}
.\end{align*}