Deductions from the Given Information

We can observe that there are two triangles here, $RSU$ and $TSU$.

Firstly, we can work out that their bases are equal:

$RS$ is the base of triangle $RSU$
$ST$ is the base of triangle $TSU$
Since we are told that $RS=ST$, we know that both triangles $RSU$ and $TSU$ have equal bases.

Secondly, we can work out that their perpendicular heights are equal:

The height of triangle $RSU$ is the perpendicular distance from the line $RS$ to the point $U$
The height of triangle $TSU$ is the perpendicular distance from the line $ST$ to the point $U$
We can see that the lines $RS$ and $ST$ are at the same vertical location, and point $U$ is also at a fixed vertical location.
So, the heights of $RSU$ and $TSU$ must be the same.

Finally, we can work out that their areas are equal:

$Triangle\,Area = Base * Perpendicular\,Height$

Since both $RSU$ and $TSU$ have the same base and perpendicular height, their areas are equal.

Solving the Problem

Since the two quantities we are told to compare are the areas of $RSU$ and $TSU$, which we have already calculated, we know that they are equal.

So, the answer would be C.