To solve this problem, we need to determine the percentage of Thymine, Guanine, and Cytosine in a DNA molecule where Adenine makes up 30%.$\newline$According to Chargaff's rules, in a DNA molecule:$\newline \bullet$The amount of Adenine (A) is equal to the amount of Thymine (T).$\newline \bullet$The amount of Guanine (G) is equal to the amount of Cytosine (C).$\newline$Given that Adenine (A) is 30%, Thymine (T) will also be 30%.$\newline$Since the total percentage of all bases in DNA must equal 100%, we can calculate the combined percentage of Guanine (G) and Cytosine (C) as follows:$\newline$Total percentage of A and T = 30% + 30% = 60%$\newline$Therefore, the combined percentage of G and C = 100% - 60% = 40%$\newline$Since G = C, each will be half of 40%, which is 20%.$\newline$Thus, the percentages are:$\newline \bullet$Thymine (T): 30%$\newline \bullet$Guanine (G): 20%$\newline \bullet$Cytosine (C): 20%$\newline$This corresponds to Option 3: T : 30 ; G : 20 ; C : 20.