There are two mechanisms of regulating renal blood flow, the myogenic mechanism and tubulo-glomerular feedback. This question asks purely about the myogenic mechanism, which is where the afferent arteriole controls blood flow based purely off blood pressure entering the kidney, which is why decreased afferent arteriolar resistance is the best answer (the arteriole is dilating in response to the decreased blood flow in attempt to maintain normal blood flow to the kidney).
Myogenic mechanism works in the afferent arteriole that supplies the glomerulus. When blood pressure increases, smooth muscle cells in the wall of the arteriole are stretched and respond by contracting to resist the pressure, resulting in little change in flow. When blood pressure drops, the same smooth muscle cells relax to lower resistance, allowing a continued even flow of blood.
Click the link to read about tuburoglomerular feedback as well.
To add to what everyone else has said, this slide is pretty good: https://images.slideplayer.com/32/10012453/slides/slide_2.jpg
the prompt says RPF and GFR are unchanged, thus RBF is also unchanged, to do that, the renal arteries would have to decrease their resistance.
However, as shown on page 571 of FA19, a constriction or dilation of efferent would change FF which means RPF and GFR aren't changing equally - this is ruled out by the prompt saying they're unchanged.
Leaves us with a decrease in afferent resistance.