SIADH is characterized by :

*Excessive free water retention ( less water in urine means specific gravity increases and not decreases as in option E)

*Euvolemic hyponatremia with continued urinary Na+ excretion ( and so urine Na which actually be greater than and not less than 10 mEq/L option D)

*Urine osmolality > serum osmolality ( option C)

Serum potassium does not actually decrease ( counter-intuitively ) because the excess fluid retention suppresses aldosterone secretion . The two main stimuli for potassium loss/secretion in kidneys are - aldosterone - and high urine flow rates. Both of these are decreased in SIADH ( excluding option B )

Serum urea nitrogen and creatinine are diluted , so their concentration decreases

In SIADH, the excessive ADH causes the collecting duct of the kidney to reabsorb huge amounts of water that it should normally excrete. That means that the plasma will now have much more water relative to solute (low osmolality) and the urine will have much more salt relative to water (higher osmolality).