The Body's Buffer System

"The bicarbonate buffering system is an important buffer system in the acid-base homeostasis of living things, including humans. As a buffer, it tends to maintain a relatively constant plasma pH and counteract any force that would alter. In this system, carbon dioxide $(\text{CO}_2)$ combines with water $(\text{H}_2\text{O})$ to form carbonic acid $(\text{H}_2\text{CO}_3)$ , which in turn rapidly dissociates to form hydrogen ions $(\text{H}^+)$ and bicarbonate $(\text{HCO}^{3-})$ as shown in the reactions below.

$\rm CO_2 + H_2O \rightleftarrows H_2CO_3 \rightleftarrows HCO_3^- + H^+$

The carbon dioxide - carbonic acid equilibrium is catalyzed by the enzyme carbonic anhydrase; the carbonic acid - bicarbonate equilibrium is simple proton dissociation/association and needs no catalyst."

Source: Wikipedia -- Bicarbonate buffering system

Of the four choices, only $\text{HCO}_{3}^{-}$ is alkaline and found in blood. The reaction is as follows:

$2\text{H}^{+} + \text{CO}_{3}^{2-} \rightleftharpoons \boxed{\text{H}^{+} + \text{HCO}_{3}^{-} \rightleftharpoons \text{H}_{2}\text{CO}_{3} + \text{H}_{2}\text{O}}$

I didn't 'know' the answer, so much as I knew that a negative ion would help balance an acid solution and there was only one choice in that regard.