The correct answer is $\boxed{\frac{3}{4}}$.
The probability that max [X, Y] is less than $\frac{1}{2}$ is the same as the probability that both $X$ and $Y$ are less than $\frac{1}{2}$. The probability that $X$ is less than $\frac{1}{2}$ is $\frac{1}{2}$, and the probability that $Y$ is less than $\frac{1}{2}$ is also $\frac{1}{2}$, since $X$ and $Y$ are independent. Therefore, the probability that max [X, Y] is less than $\frac{1}{2}$ is $\frac{1}{2} \times \frac{1}{2} = \boxed{\frac{1}{4}}$.
Here is a more detailed explanation of each option:
- Option A: $\frac{3}{4}$. This is the correct answer. The probability that max [X, Y] is less than $\frac{1}{2}$ is the same as the probability that both $X$ and $Y$ are less than $\frac{1}{2}$. The probability that $X$ is less than $\frac{1}{2}$ is $\frac{1}{2}$, and the probability that $Y$ is less than $\frac{1}{2}$ is also $\frac{1}{2}$, since $X$ and $Y$ are independent. Therefore, the probability that max [X, Y] is less than $\frac{1}{2}$ is $\frac{1}{2} \times \frac{1}{2} = \boxed{\frac{1}{4}}$.
- Option B: $\frac{9}{{16}}$. This is not the correct answer. The probability that max [X, Y] is less than $\frac{1}{2}$ is the same as the probability that both $X$ and $Y$ are less than $\frac{1}{2}$. The probability that $X$ is less than $\frac{1}{2}$ is $\frac{1}{2}$, and the probability that $Y$ is less than $\frac{1}{2}$ is also $\frac{1}{2}$, since $X$ and $Y$ are independent. Therefore, the probability that max [X, Y] is less than $\frac{1}{2}$ is $\frac{1}{2} \times \frac{1}{2} = \boxed{\frac{1}{4}}$. This is not equal to $\frac{9}{{16}}$.
- Option C: $\frac{1}{4}$. This is not the correct answer. The probability that max [X, Y] is less than $\frac{1}{2}$ is the same as the probability that both $X$ and $Y$ are less than $\frac{1}{2}$. The probability that $X$ is less than $\frac{1}{2}$ is $\frac{1}{2}$, and the probability that $Y$ is less than $\frac{1}{2}$ is also $\frac{1}{2}$, since $X$ and $Y$ are independent. Therefore, the probability that max [X, Y] is less than $\frac{1}{2}$ is $\frac{1}{2} \times \frac{1}{2} = \boxed{\frac{1}{4}}$. This is not equal to $\frac{1}{4}$.
- Option D: $\frac{2}{3}$. This is not the correct answer. The probability that max [X, Y] is less than $\frac{1}{2}$ is the same as the probability that both $X$ and $Y$ are less than $\frac{1}{2}$. The probability that $X$ is less than $\frac{1}{2}$ is $\frac{1}{2}$, and the probability that $Y$ is less than $\frac{1}{2}$ is also $\frac{1}{2}$, since $X$ and $Y$ are independent. Therefore, the probability that max [X, Y] is less than $\frac{1}{2}$ is $\frac{1}{2} \times \frac{1}{2} = \boxed{\frac{1}{4}}$. This is not equal to $\frac{2}{3}$.