In order for this code to represent a piece of dna, which base must be replaced by thymine?

I guess you forgot to add the picture or so.
Based on the Complementarity of the 4 nucleotides, The thymine must always be placed in  front of the Adenine, and the Cytosine must always be placed in front of the Guanine.
Any change in this rule causes the deformation of the DNA, and can sometimes cause fatal diseases like the melanoma (skin cancer) etc...

Hope this Helps! :)