Concept: The question tests on the basics of Percentage and Fractions.
Let there be "x" flowers
Number of roses = 40% = 0.4x
Number of carnations = 100-40=60% = 0.6x
1/4 of the roses = 1/4 * (0.4x) = 0.1x and
1/10 of carnations = 1/10 * (0.6x) = 0.06x are RED
Probability that a red flower selected at random is a rose= 0.1x / 0.16x
= 5/8 (option b)