Q. Which substance is a base? \(\text{HCOOH}\), \(\text{RbOH}\), \(\text{H}_2\text{CO}_3\), \(\text{NaNO}_3\).

We first identify which choices are bases. In general chemistry, a base is a substance that can accept a proton (often giving rise to a hydroxide, \( \mathrm{OH^-} \), in water) or that produces basic ions in solution.

We are given: \(\mathrm{HCOOH}\), \(\mathrm{RBOH}\), \(\mathrm{H_2CO_3}\), \(\mathrm{NaNO_3}\).

Now we check each one.

1) \(\mathrm{HCOOH}\) (formic acid)

\(\mathrm{HCOOH}\) is an acid because it contains an acidic hydrogen attached to a carbonyl-containing group. Acids can donate a proton, so this substance is not a base.

Conclusion: \(\mathrm{HCOOH}\) is an acid, not a base.

2) \(\mathrm{RBOH}\)

\(\mathrm{RBOH}\) has the pattern \(\mathrm{MOH}\) (a metal hydroxide type). Hydroxides contain \(\mathrm{OH^-}\), which is the key basic species.

In water, hydroxides are sources of \(\mathrm{OH^-}\), so they are bases.

Conclusion: \(\mathrm{RBOH}\) is a base.

3) \(\mathrm{H_2CO_3}\) (carbonic acid)

\(\mathrm{H_2CO_3}\) is an acid because it contains acidic hydrogen atoms and can donate \( \mathrm{H^+} \) in water.

Conclusion: \(\mathrm{H_2CO_3}\) is an acid, not a base.

4) \(\mathrm{NaNO_3}\) (sodium nitrate)

\(\mathrm{NaNO_3}\) is a salt made from a strong base (\(\mathrm{NaOH}\)) and a strong acid (\(\mathrm{HNO_3}\)). Such salts are typically neutral in water.

It does not contain \(\mathrm{OH^-}\) and is not a base.

Conclusion: \(\mathrm{NaNO_3}\) is not a base (it is neutral).

Final Answer: The substance that is a base is \(\mathrm{RBOH}\).