Answer:
In order to obtain the rectangular cross-section on this cake we need to do slicing:
Parallel to the base.
Explanation:
We know that when slicing is done parallel to the base than we get a cross-section that is similar to the base ( i.e. of same shape and size).
Hence, here we have a cake in shape of a rectangular prism or we may say a cuboid so in order to get the rectangular cross-section we need to cut this cake parallel to the base.
Hence, the answer is:
Parallel to the base.