Answer
$$F'(t)=\coth t$$
Work Step by Step
$F'(t)=\frac{d}{dt}\ln{(\sinh t)}$
Using the chain rule:
$F'(t)=\frac{d\ln{(\sinh t)}}{d\sinh t}
\times \frac{d\sinh t}{dt}$
$=\frac{1}{\sinh t} \times \cosh t$
$=\frac{\cosh t}{\sinh t}$
$=\coth t$