To find the volume of a rectangular prism, you can use the formula:
\[ \text{Volume} = \text{length} \times \text{width} \times \text{height} \]
In this case, since Rhonda used unit cubes to create the prism, the volume will be equal to the total number of unit cubes used.
If the total number of unit cubes used is not directly mentioned, you can typically assume that the volume is the sum of the provided options.
The provided options are:
- 14 cubic units
- 21 cubic units
- 42 cubic units
- 63 cubic units
However, without specific dimensions to calculate the volume from, I cannot determine the exact volume.
If Rhonda indeed used all the cubes that resulted in a combination or arrangement equating to any of the options given, you will need to provide more information about the individual dimensions of the prism or specify which of these volumes apply.
Alternatively, if you are asking for the largest possible volume she could use based on these options, it would be 63 cubic units, assuming that she used the maximum quantity of unit cubes for her rectangular prism.