Aaah, in this question, the clue that you should look at, is the phrase "how much water it can hold?"
If the statement is like that, you can be sure that it is talking about "volume."
So here it is:
Given: Rectangular aquarium (rectangular prism, if am not mistaken) = shape
Length = 16 dm
Width = 15 dm
Height = 10 dm
Required: How much water can it hold?
Formula: Volume of Rectangular Prism = Length x Width x Height
Mathematical Sentence: V = L x W x H
V = 16 dm x 15 dm x 10 dm
(use the left-to-right method in this one)
V = 240 dm^2 x 10 dm
V = 2, 400 dm ^3
The rectangular aquarium can hold 2, 400 dm^3 of water.
The volume of the rectangular aquarium is 2, 400 dm^3 of water.