Step 1: Recall where the word brine comes from.
Brine is simply the everyday chemistry term for salty water, the same salt you use in your kitchen dissolved in water in a strong concentration.
Step 2: Write its make up.
That kitchen salt is sodium chloride, $NaCl$, so brine is nothing but a concentrated aqueous solution of $NaCl$. It is not sodium hydroxide, sodium carbonate or sodium bicarbonate dissolved in water, those are different sodium compounds entirely.
Step 3: Confirm with its main use.
Brine is fed into the chlor alkali cell, where electrolysis splits it apart. \[ 2NaCl + 2H_2O \xrightarrow{\text{electrolysis}} 2NaOH + Cl_2\uparrow + H_2\uparrow \] Notice $NaOH$ is a product here, not brine itself, which confirms brine could not already be $NaOH$ solution.
Step 4: Conclude.
Brine is an aqueous solution of sodium chloride. \[ \boxed{\text{Aqueous solution of NaCl}} \]