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Solution:

Ans. Option 1) is correct: 15% A, 15% T, 30% G, 40% C

Chargaff's rule tells that the number of purine should be equal to number of pyrimidine.

i.e.  Purine (Adenine + guanine) = Pyrimidine (Cytosine + Thymine)
 

Complementary base pairs in DNA are those pairs of nucleotides which show pairing as suggested by Watson and Crick. 

• Adenine of one strand pairs with Thymine of another strand by two hydrogen bonds and vice versa. (A=T or T=A). Guanine of one strand pairs with Cytosine of another strand by three hydrogen bonds and vice versa. G = C or C = G
• A purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.