Probiotics are live bacteria and yeasts that are beneficial for the human body, particularly for the digestive system. They are often recommended to patients who are on prolonged illness and antibiotic regimes because the antibiotics may kill the beneficial bacteria in the gut along with the harmful ones.

The correct answer is option 3: lactic acid bacteria. This refers to a type of bacteria that naturally produces lactic acid as a byproduct of their metabolism. Lactic acid bacteria are one of the most common types of probiotics found in various supplements and fermented foods like yogurt and kimchi.

Option 1 (proteins) is not correct because while probiotics may contain certain proteins, they are not the main component or purpose of probiotic supplements.

Option 2 (vitamins) is also not correct because although some probiotic supplements may contain small amounts of vitamins, it is not their primary function.

Option 4 (electrolytes) is not correct because probiotics do not typically contain electrolytes, which are substances that conduct electrical signals in the body.

Therefore, the correct answer is option 3: lactic acid bacteria, as these bacteria are commonly found in probiotic supplements and have beneficial effects on the digestive system when consumed.