What electrolytes do the kidneys regulate?

Hint: Electrolytes are charged particles or elements (cations or anions) that may serve or help in various physiological and metabolic functions of the body. These electrolytes are taken along with diet and are excreted regularly on a controlled basis by the kidneys. The rate of excretion depends on the rate of intake of electrolytes.
Any type of complication like failure of excretion of these electrolytes or excessive excretion may lead to complications in the body.

Complete answer:
The common electrolytes whose excretion is regulated by the kidneys are:
Sodium: This is a major electrolyte of the body which exists as Na+. Its major function is in nerve and muscle physiology. It is taken up into the body with fluids and diet and excreted majorly in urine and through sweat.
The Kidneys normally maintain a balance in the concentration of sodium in the body. Failure of the kidneys to do so may result in hypernatremia and hyponatremia.
Potassium: The potassium exists in the body as K+. It is required from the normal functioning of cells and has a marked role in nerve and muscular physiology.
Failure of the kidneys to control its excretion may result in hyperkalemia and hypokalemia.
Phosphorus: It exists in the body in the phosphate ions ($PO_4^{3-}$, $PO^{3-}$). Failure of the kidneys in its proper excretion may result in hyperphosphatemia and hypophosphatemia.
Calcium: This electrolyte exists in the body in the form of $Ca^{2+}$. This is a very important electrolyte that is involved in many functions such as formation of bones and teeth, in muscular contraction, neurotransmitter release from nerve endings, coagulation of blood, etc.
The regulation of this electrolyte is mainly controlled by two hormones- Calcitonin and Parathyroid hormone.
Small amounts are excreted which depends on the action of these two hormones.
Magnesium: Magnesium exists in the body in the form of $Mg^{2+}$. Very small amounts are excreted in the urine, but even if the amount is small, then also it is vital in the regulation.
Failure of regulation of magnesium levels may lead to hypermagnesemia and hypomagnesemia.

Note:
Some few noteworthy points are:
Renal Failure is often complicated by elevated potassium, phosphate and magnesium and decreased sodium and calcium.
Not all electrolytes are excreted or regulated by the kidneys. For example, manganese and iron are also essential electrolytes but these are not regulated by the kidneys.
Some of the electrolytes and other substances that get filtered out through the glomerular tuft in the nephron are reabsorbed as the filtrate passes through the PCT, loop of Henle and DCT portions.
This reabsorption depends on a value popularly known as threshold value. The substance which has a high threshold will be reabsorbed more through the nephron and the ones with a low threshold value will be eliminated through urine. E.g., glucose is a high threshold substance.
The electrolyte concentration is regulated by the kidneys by altering the threshold values from time to time, which is due to a series of chemical dialogues and another topic of discussion.