How many amino acids could be specified if codons consisted of two nucleotides instead of three? what problem would this present for an organism making proteins using all 20 amino acids?

The correct answer would be 16.

It can be explained as:

There are only four ribonucleotides present in an organism which are used to form a codon.

The four ribonucleotides are adenine, guanine, cytosine, and uracil.

If the codons consisted of two nucleotides instead of three then the possible number of combinations would be 4² that is, 16.

Thus, a maximum of 16 amino acids could be specified.

The organism would not be able to form proteins using all 20 amino acids.  

It is why three nucleotides are used to form a codon instead of two.

i think that it is plants