Answer
$e$
Work Step by Step
A natural logarithm is a logarithm to base $e$. As shown on page 582, when a natural logarithm is written in the form $ln (x)$, this is equivalent to the expression $log_{e}x$.
Therefore, $ln 7$ is a natural logarithm that can also be written as $log_{e}7$. So the base of $ln 7$ is $e$.
