The following postfix expression with single digit operands is evaluated using a stack:

$\frac{{823}^{{?}^{?}}}{23}**+51**-$

Note that ${?}^{{?}^{{?}^{?}}}$ is the exponentiation operator. The top two elements of the stack after the first ${?}^{**}$ is evaluated are:

Your answer:

$3,2$

$6,1$

$1,5$

$5,7$