The correct answer is False. Standard normal random variables are not always labeled as $Z$. In fact, they can be labeled with any letter or symbol, as long as it is clear from the context that the variable is standard normal. For example, we could write $X$, $Y$, or $Z$ to represent a standard normal random variable.
The reason why standard normal random variables are sometimes labeled as $Z$ is because the standard normal distribution is often denoted by $\mathcal{N}(0,1)$. This means that a standard normal random variable has a mean of 0 and a standard deviation of 1.
However, it is important to remember that the standard normal distribution is just one type of normal distribution. There are many other types of normal distributions, each with its own mean and standard deviation. Therefore, it is not always appropriate to label a normal random variable as $Z$.