It's Sonic

Firstly, the speed of sound:

$c=f\lambda=34000\,\mathrm{s}^{-1}\times0.01\,\mathrm{m}=340\,\mathrm{ms^{-1}}.$

Now, let $t\,\mathrm{s}$ be the time between the pulse being emitted and received. Consider the distance $d\,\mathrm{m}$ travelled by the bat and by the pulse in this time:

$d_{\mathrm{bat}}=18t;$ $d_{\mathrm{pulse}}=340t.$

We know that the pulse travels $3.78\,\mathrm{m}$ to the wall, and $(3.78-d_{\mathrm{bat}})\,\mathrm{m}$ back to the bat. So:

$d_{\mathrm{pulse}}=3.78+(3.78-d_{\mathrm{bat}}),$

i.e.

$340t=3.78+(3.78-18t)$

$\Rightarrow358t=7.56$

$\Rightarrow t=7.56\div358=0.0211\dots$

So, to the nearest millisecond, the time taken for the pulse to return to the bat is

$\boxed{21\,\mathrm{ms}}.$