Consider the incomplete reaction below.
naoh + x mc005-1.jpg nach3coo + h2o
what is x in this reaction?
nh4oh
h3po4
h2co3
ch3cooh

First, we can see that the output is a salt and water which means that the reactants were an acid and a base. Since the given reactant is a base, thus, NH4OH is excluded as it is also a base.

Now, examining the outputs of the reactions, we will find that they are composed mainly of sodium, hydrogen, oxygen and carbon. This excludes H3PO4 as it contains phosphorus.

The last step is to compare between the number of moles of each element in the output with those in the input. The output has 1 sodium mole, 2 carbon moles, 5 hydrogen moles and 3 oxygen moles.
The given input (NaOH) has 1 sodium, 1 oxygen and 1 hydrogen moles. Therefore, the other input (reactant) should have 2 carbon, 4 hydrogen and 2 oxygen moles.

Based on this, the right solution is CH3COOH.