Increased renal blood flow is responsible for the increased urinary output during pregnancy.

This is because the renal plasma flow and glomerular filtration rate increase by 50% to 80% during pregnancy, resulting in more urine production.

Choice A is wrong because the glomerular filtration rate increases, not decreases, during pregnancy.

Choice B is wrong because the tubular reabsorption decreases, not increases, during pregnancy.

This is due to the increased levels of progesterone, which inhibit sodium reabsorption and cause diuresis.

Choice D is wrong because the increased plasma volume is a consequence, not a cause, of the increased renal blood flow.

The plasma volume expands by 30% to 50% during pregnancy to meet the increased metabolic demands of the mother and fetus.