Which cross will yield four phenotypes in the 1: 1: 1: 1 ratio

The answer is RrYy x rryy.

It is known that the in dihybrid cross (a cross between two different traits), the phenotypic ratio between the offspring in the F2 generation is 9:3:3:1. To get phenotypes in ratio 1:1:1:1, there must be a cross between heterozygote for both traits and recessive homozygote for both traits. This can be concluded if these two traits are analyzed separately:
Parents:       Rr    x    rr
Offspring: Rr    Rr   rr    rr
So, the ratio is 2:2 or 1:1.
The same ratio is for cros Yy x yy.
Thus, the cross between RrYy and rryy will yield four phenotypes in the 1:1:1:1 ratio.
In the attached photo, 1/4 of the offspring are RrYy, 1/4 are Rryy, 1/4 are rrYy, and 1/4 rryy. The ratio is 1/4:1/4:1/4:1/4, which is the same as 1:1:1:1.