A DNA fragment contains 18% Thymine(T). The combined content of Cytosine (C) and Guanine (G) in it would be:

64%

The correct answer is Option ( 3 ) - 64%

Chargaff's rule of DNA base pairing asserts that if we have knowledge of the DNA sequence on one strand, we can readily infer the nucleotide base pairs on the complementary strand. DNA nucleotides consist of purines and pyrimidines, with Adenine (A) and guanine (G) classified as purines, and thymine (T) and cytosine (C) as pyrimidines. According to this rule, Adenine always pairs with thymine, while cytosine pairs with guanine, forming hydrogen bonds between complementary nucleotides.
According to Chargaff’s rule in a DNA molecule A = T and G = C and A + G + C + T = 100%

In other words, A + T = C + G. This implies that the number of adenine must match the number of thymine, and concurrently, the number of cytosine must match the number of guanine.

In the provided question, it is mentioned that the percentage of thymine is 18%. Applying the above logic, this implies that the percentage of adenine is also 18%.

To maintain the balance of A + T = C + G, let's assume the total percentage of A + T + C + G to be 100%. Subtracting the percentages of thymine (18%) and adenine (18%) from 100 leaves us with 64%.

Since the percentages of cytosine and guanine, must be the remaining 64%.