Z-transform of not quite an upsampler
Ok, its a trick question...its $Y(z) = X(z)$ because $y[n]$ and $x[n]_L$ are identical sequences.

$$
y[n] = \begin{cases}
x[n]_L &amp; 0 \le n \le L-1\\
0&amp;otherwise
\end{cases} 
\longrightarrow  Y(z) = X(z)
$$