Split up the interval [2, 5] into

equally spaced subintervals, then consider the value of

at the right endpoint of each subinterval.
The length of the interval is

, so the length of each subinterval would be

. This means the first rectangle's height would be taken to be

when

, so that the height is

, and its base would have length

. So the area under

over the first subinterval is

.
Continuing in this fashion, the area under

over the

th subinterval is approximated by

, and so the Riemann approximation to the definite integral is

and its value is given exactly by taking

. So the answer is D (and the value of the integral is exactly 39).
Let's talk about the choices
◯ ∠4 and ∠5 . . . . . . . . these are complementary angles whose sum is 90°
◯ ∠7 and ∠9 . . . . . . . . these are vertical angles, equal to each other
◯ ∠2 and ∠3 . . . . . . . . these are supplementary angles that together form a straight line. They are a linear pair.
◯ ∠1 and ∠2 . . . . . . . . these are "adjacent" angles in the quadrilateral that also includes ∠9 and ∠10.
The appropriate choice is ...
◉ ∠2 and ∠3
Most linear systems you will encounter will have exactly one solution. However, it is possible that there are no solutions, or infinitely many. (It is not possible that there are exactly two solutions.) The word unique in this context means there is a solution, and it's the only one.
Hope this helps.