In this question the aircraft is in the cruise with a coarse pitch. If the rpm lever is pulled rearwards the pitch will increase further. This will reduce the drag and thus the L/D ratio, reducing the glide angle and decreasing the rate of descent.

This is what the question asks, but it is useful to go a stage further and identify the effect on the A of A. As the aircraft is descending at a constant IAS in the glide the TAS will decrease and the A of A will increase.