Which among the following is a base?

NaOH

The correct answer is option 3. HCl.

Sodium hydroxide (NaOH) is a base. Bases are substances that can accept protons (H⁺ ions) or donate hydroxide ions (OH⁻) in aqueous solutions. Sodium hydroxide dissociates in water to produce hydroxide ions:

\[ NaOH \rightarrow Na^+ + OH^- \]

The hydroxide ion (OH⁻) is what gives sodium hydroxide its basic properties. In solution, hydroxide ions can react with acids to form water and a salt. For example, with hydrochloric acid (HCl):

\[ NaOH + HCl \rightarrow NaCl + H_2O \]

In this reaction, the hydroxide ion (\(OH^-\)) from sodium hydroxide combines with the hydrogen ion (\(H^+\)) from hydrochloric acid to form water (\(H_2O\)), and sodium chloride (\(NaCl\)) is produced as a salt.

The other compounds mentioned:

NaCl (sodium chloride): This is a salt, not a base.

HCl (hydrochloric acid): This is an acid.

MgCl₂ (magnesium chloride):  This is a salt, not a base.

In summary, NaOH is a base because it produces hydroxide ions in solution, and it can neutralize acids through the reaction with hydroxide ions.