Most land vertebrates can't drink seawater because
a. their kidneys can't excrete concentrated salt.
b. less water would need to be excreted by the kidneys.
c. they don't have kidneys.
d. their cells would eventually swell.

D) Is incorrect because water would be moving out of the cell. It would be in a hypertonic solution, causing it to shrivel.

C) Is incorrect because land vertebrates have kidneys

B) Is incorrect because when salt is consumed in a larger quantity (salt water) the body will signal the kidneys to filter out more salt - dehydrating the organism. Essentially, the body would need to excrete more water/urine

So, answer (A) is correct.
Due to the kidney's inability to filter out so much salt, the kidneys have a larger water output (which dilutes the salts).