The basic solution which does not contain a metallic element is:
A. Sodium oxide
B. Ammonia
C. Potassium hydroxide
D. Ammonium hydroxide

Hint: When the volume of a sample is increased by just the addition of water then the process is termed as dilution. The dilution not only increases the volume of the sample but decreases the concentration of the sample too. Upon dilution since the concentration of the sample decreases it may start to show properties if the dilute version of the sample and that may affect its dissociation power as a whole.

Complete step by step answer:
Sodium oxide, when dissolved in water, when dissolved in water will result in the formation of sodium hydroxide which will contain the metal ion of sodium metal which is not needed so it cannot be the correct option.
Ammonia, when dissolved in water, when dissolved in water will ultimately form the basic solution of ammonium hydroxide
When potassium hydroxide is added to water to form basic products then it leads to the formation of the solution but it will also contain the metallic ion is potassium so this can also not be the answer of the question asked.
An aqueous solution of ammonia gas which is formed upon the dissolution of ammonia in water in the form of basic ammonium hydroxide aqueous solution which does not have a metallic element.

So, the correct answer is Option D.

Note: The $pH$ of a sample can be tested physically through many ways like the Litmus paper and the $pH$ paper.
The $pH$ paper gives different colours based on the different $pH$ of different samples while the litmus paper only allows us to know if the sample is either basic or acidic. It changes the colour to blue if the sample is basic while it changes to red in case of an acidic sample.