The most basic amino acid among the options given is arginine. Amino acids are organic compounds that are the building blocks of proteins. They contain both an amine group (-NH2) and a carboxyl group (-COOH), which are attached to a central carbon atom called the alpha carbon. Additionally, there is a variable side chain, represented by the letter R, that differentiates each amino acid.

Arginine has a guanidino group in its side chain, which contains a nitrogen atom. This guanidino group has a lone pair of electrons on the nitrogen atom, making it a strong base. The lone pair of electrons can accept a proton, resulting in the formation of a positive charge on the nitrogen atom. This makes arginine a basic amino acid.

Let's compare the side chains of the other amino acids mentioned in the options:

a) Tyrosine: Tyrosine has a phenol group in its side chain, which contains an -OH group. While -OH groups can act as weak bases, they are not as basic as the guanidino group in arginine.

b) Methionine: Methionine has a sulfur atom in its side chain, which is not directly involved in basicity. Therefore, methionine is not as basic as arginine.

d) Glutamine: Glutamine has an amide group (-CONH2) in its side chain, which does not have any basic properties. Therefore, glutamine is not as basic as arginine.

In summary, among the given options, arginine is the most basic amino acid due to the presence of a guanidino group in its side chain.