Answer
Answer Choice $F$: $\frac{q \times u}{s}$
Work Step by Step
Since. $\triangle$$ABC$ and $\triangle$$DEF$ are similar,
that means that $\frac{s}{q} = \frac{t}{r} = \frac{u}{?}$.
With the equation $\frac{t}{r} = \frac{u}{?}$, if you cross multiply, you get
$? \times t = u \times r$
If we divide both sides by $t$, we get
$? = \frac{u \times r}{t}$
With the equation $\frac{s}{q} =\frac{u}{?}$, if you cross multiply, you get
$u \times q$ = $s$ $\times$ $?$
If you divide both sides by $s$, you get
$?$ = $\frac{u \times q}{s}$
The answer is either $\frac{u \times r}{t}$ or $\frac{u \times q}{s}$
Answer choice $F$ shows $\frac{q \times u}{s}$, which is the same thing as $\frac{u \times q}{s}$, so that is the answer.
