Past realized rate of return in period t is denoted by

The correct answer is: $\boxed{\text{B. }t\hat{r}}$

The past realized rate of return in period $t$ is denoted by the symbol $t\hat{r}$. This is because the past realized rate of return is a historical fact, and it is estimated by taking the ratio of the ending value of an asset to its beginning value, and then subtracting 1. For example, if the ending value of an asset in period $t$ is $100$ and its beginning value in period $t$ is $90$, then the past realized rate of return in period $t$ is $100/90-1=11.11\%$.

The other options are incorrect because:

• Option A, $t\bar{r}$, is not a standard way of denoting the past realized rate of return.
• Option C, $\hat{r}t$, is not a standard way of denoting the past realized rate of return.
• Option D, $r\bar{t}$, is not a standard way of denoting the past realized rate of return.