Which property can be justified using the ratios in triangle XYZ

The correct answer is B. 

We can actually tell this without even doing work as every answer for Cos(α) = Sin(90 - α) due to the way that Sin and Cos wave behave. However, we can also tell this by using some mathematical evaluation. 

Sin(Y) = Cos(90 - Y)
Sin(50) = Cos(90 - 50)
.766 = Cos(40)
.766 = .766

This should that they are exactly the same.