The nephrons have LOHs that span deeply into the medulla. The is the capillary bed that drains these extended LOHs.
a. Cortical; vasa recta
b. Cortical; peritubular
c. Juxtamedullary; peritubular
d. Juxtamedullary; vasa recta

The preferable option for the fill in the blanks will be - D.

D. Juxtamedullary; vasa recta

Explanation:

The juxtamedullary nephrons have LOHs that span deeply into the medulla. The vasa recta are the capillary bed that drains these extended LOHs.

LOH = loop of Henle  

We know that the longitudinal section of the kidney has two regions - 1. an outer dark cortex and 2. inner light medulla. The juxtamedullary nephron is the structural and functional unit of the kidney with a long loop of Henle.  Juxtamedullary nephrons have a long loop of Henle extending deep into the renal medulla. The vasa recta capillaries are long, hairpin-shaped blood vessels that run parallel to the loops of Henle. The hairpin turns slow the rate of blood flow, which helps maintain the osmotic gradient required for water reabsorption.

Mg is used to measure mass