Don't worry, just write it out.
[tex]$\frac{15xy^7z^4}{5x^6yz^{16}} = \frac{15}5 \cdot \frac x{x \cdot x \cdot x \cdot x \cdot x \cdot x} \cdot \frac{y \cdot y \cdot y\cdot y\cdot y \cdot y \cdot y}{y} \cdot \frac{z^4}{z^{16}}$[/tex]
Note that the top x cancels with one of the bottom x's, and on the other fraction, the bottom y cancels with one of the top y's. For the z's, using the same reasoning, the four z's on the top cancel with four on the bottom, leaving 16 - 4 = 12 z's on the bottom. So you get
[tex]$3 \cdot \frac1{x \cdot x \cdot x\cdot x\cdot x} \cdot (y \cdot y \cdot y \cdot y \cdot y \cdot y) \cdot \frac 1{z^{12}}$[/tex]
which is
[tex]$3 \cdot \frac1{x^5} \cdot y^6 \cdot \frac 1{z^{12}}$[/tex]
[tex]$= \boxed{\textbf{D) }\frac{3y^6}{x^5z^{12}}}$[/tex]
