The descending limb of the loop of henle
a. pulls water by osmosis into the lumen of the tubule
b. is freely permeable to sodium and urea
c. is not permeable to water
d. contains fluid that becomes more concentrated as it moves down into the medulla

D. contains fluid that becomes more concentrated as it moves down into the medulla

Explanation:

In kidney, loop of Henle is portion of the nephron which leads from proximal convoluted tubule to distal convoluted tubule.

Liquid entering loop is  solution of the salt, urea, and also other substances which are passed along by proximal convoluted tubule. It is the tubule from which most of dissolved components which are needed by body are reabsorbed into blood. First segment of loop,descending limb, is permeable to the water, and liquid reaching bend of loop is much richer than blood plasma in the salt and the urea. As liquid returns through ascending limb, NaCl diffuses out of tubule into surrounding tissue, where concentration is lower. In third segment of loop, tubule wall effect further removal of the salt.

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.

well, i believe that the answer is 2. : 3