I think I have this figured out. Had to do some math. With our 4 nucleotide system, the number of possible codons is 43 = 64. That is number of nucleotides to the codon length power. The question asks us to determine codon length while giving us the number of amino acids and an upper limit on tRNAs. Using the same formula,
#Nucleotides(Codon-Length) = #Codons,
we must find a combination of nucleotide number and codon length that gives between 20 and 40 codons.
Nucleotides 1 2 3 4 5
2 2 4 8 16 32
3 3 9 27 81 243
4 4 16 64 256 1024
5 5 25 125 625 3125
6 6 36 216 1296 7776
7 7 49 343 2401 16807
So using this table we see that a few different combinations of Nucleotide number and codon length will fit in that range. A 2-nucleotide system could work if each codon was 5 bases long. A 3-nucleotide system could work with 3 base long codons. A 4-base system cannot work, it goes from less than 20 to more than 40. A 5-base system can work with 2 base codons. A 6-base system can also work with 2-base codons. 7 or more bases fails because even a 2-base codon will make more than 40 codons.
A codon length of 1 nucleotide would require at least 20 nucleotides, so it would not be a good answer.
5 is probably the correct answer because it works with the shortest codon length, though I don't know how you'd get base pairing with an odd number of bases. Perhaps the 5th base pairs with itself. A 6 base system can also work with the same codon length and wouldn't need a self pairing base. However that table shows several combinations could work, so this question might have been to see how you defend your answer.