Which of the following substance has a pH value of about 14?

Sodium hydroxide

The correct answer is option 2. Sodium hydroxide.

The pH scale measures the acidity or basicity (alkalinity) of a solution. It ranges from 0 to 14, with 7 considered neutral. A pH less than 7 indicates acidity, while a pH greater than 7 indicates alkalinity. The scale is logarithmic, meaning that each unit change represents a tenfold difference in acidity or alkalinity.

Blood: Blood typically has a pH in the range of 7.35 to 7.45, making it slightly alkaline. It is close to neutral but slightly on the alkaline side

Sodium Hydroxide (NaOH): Sodium hydroxide is a strong base, and when dissolved in water, it dissociates to produce hydroxide ions (\(OH^-\)). The pH of a sodium hydroxide solution is highly alkaline, and it can approach the maximum value of 14 on the pH scale.

Milk of Magnesia:  Milk of magnesia, which is a suspension of magnesium hydroxide (\(Mg(OH)_2\)) in water, is alkaline but typically not as strongly alkaline as sodium hydroxide. Its pH is usually less than 14.

Lemon Juice:  Lemon juice is acidic due to the presence of citric acid. The pH of lemon juice is typically in the range of 2 to 3, making it acidic.

Therefore, among the options provided, Sodium Hydroxide (NaOH) is the substance with a pH value of about 14, indicating a highly alkaline or basic solution.