Recall that the action spectrum from Engelmann’s experiment plotted the rate of photosynthesis (as measured by oxygen production) versus wavelength. In each of the following graphs, the black line shows Engelmann’s original action spectrum deduced from the distribution of aerotactic bacteria around the alga. Which red line shows the same action spectrum corrected for the unequal number of photons emitted across the visible spectrum?

An action spectrum is ordinarily plotted so that the "activity" appeared on the y-axis is estimated with an equivalent number of photons at every wavelength of the unmistakable range. In any case, our sun doesn't emanate equivalent quantities of photons at every wavelength.
Rather, the sun transmits the most photons in the yellow piece of the range, with moderately fewer photons discharged in the red and violet-blue pieces of the range. In this manner, the red and violet-blue districts of Engelmann's action spectrum were estimated with fewer photons than in the yellow piece of the range.
To correct for this, you need to consider how expanding the quantity of photons in the red and violet-blue pieces of the range - to coordinate the emission level in the yellow part- - would change the measure of oxygen created, and therefore the number of microscopic organisms that amassed, in the red and violet-blue spectral region. The rectified action spectrum would show higher tops in the red and violet-blue pieces of the range, yet the plot in the yellow piece of the range would be around equivalent to Engelmann's.