The correct answer is $\boxed{\text{ammonia}}$.
Ammonia is the strongest base in aqueous solution because it has the highest tendency to accept a proton. This is because ammonia has a lone pair of electrons on its nitrogen atom that is not involved in bonding. This lone pair of electrons is highly electronegative, making it very attractive to protons. When ammonia accepts a proton, it forms ammonium ion, which is a strong base.
Diethylamine, triethylamine, and ethylamine are all amines, which means they have a nitrogen atom with a lone pair of electrons. However, these amines have alkyl groups attached to the nitrogen atom. The alkyl groups are electron-releasing groups, which means they withdraw electron density from the nitrogen atom. This makes the lone pair of electrons on the nitrogen atom less available to accept a proton, and therefore these amines are weaker bases than ammonia.
In conclusion, ammonia is the strongest base in aqueous solution because it has the highest tendency to accept a proton. This is because ammonia has a lone pair of electrons on its nitrogen atom that is not involved in bonding. This lone pair of electrons is highly electronegative, making it very attractive to protons. When ammonia accepts a proton, it forms ammonium ion, which is a strong base.