Answer
(a) greater than
(b) I
Work Step by Step
(a) We know that$f_{yellow}\lt f_{blue}$ and $E=nhf$. This equation shows that the number of photons emitted per second by the yellow source is greater than the number of photons emitted per second by the blue source.
(b) We know that the best explanation is option (I) -- that is, the yellow source emits more photons per second because it emits more energy per second than the blue source, and its photons have less energy than those of the blue source.
