Read the excerpt from the building of manhattan.

a temporary, small narrow-gauge track system was installed
on each floor as it was needed. this enabled the material to
be moved from the truck at ground level onto dump carts,
raised by elevator to the designated floor, wheeled onto the
track, and moved quickly to the exact spot needed.
turntables built into the track allowed the carts to be shifted
about in any direction.

which question can be answered only if an illustration
accompanies the text?

what did turntables built into the track allow?
how were dump carts raised to designated floors?
when was a track system installed on a floor?
what does a turntable built into a track look like?

What does a turntable built into a track look like?

Step-by-step explanation:

Let us go through all the choices one by one:

Choice A: What did the turntables built into the track allow?

The excerpt already tells us that "Turntables built into the track allowed the carts to be shifted  about in any direction."

Therefore this question is already answered and there is no need for an illustration.

Choice B: How were dump carts raised to designated floors?

The excerpt already tells us that "This enabled the material to

be moved from the truck at ground level onto dump carts,

raised by elevator to the designated floor, wheeled onto the

track, and moved quickly to the exact spot needed."

So we already know that the dump carts were raised to the designated floors by elevators, and therefore there is no need for an illustration.

Choice C: When was a track system installed on a floor?

An illustration is pictorial; it can only give us information that be represented by pictures or diagram. An illustration cannot give us temporal information.

Choice D: What does a turntable built into a track look like?

The except only tells us that "Turntables built into the track allowed the carts to be shifted about in any direction." It does not tell us how do these turntables look like.

Therefore, an illustration will tell us how does a turntable built into track look like. And an illustration is the only way this question can be answered.