The answer would be A) 20.
We are told that CD is a perpendicular bisector of both XY and ST. This means that the distance from MS and MT is the same, and DX and DY are also equidistant. Also, since CD is perpendicular to XY and ST, that means that ∠CDY is equal to ∠CDX and ∠CMT is equal to ∠CMS.
Therefore, since we are given CY, we can say that CX is equal to CY, and CY is equal to 20.