If complementary DNA strands were arranged in a parallel manner, what would you expect to see?
a. Complementary nucleotides would be attracted to each other, forming ionic bonds that would make the helix stable but not uniform in width.
b. Charges of complementary nucleotides would repel and no hydrogen bonds would form.
c. Complementary nucleotides would line up properly, but fewer hydrogen bonds would form so the strands could be more easily pulled apart.
d. Purines would pair with both purines and pyrimidines due to the closer alignment of the parallel strands.
e. There would be no discernable difference between DNA strands aligned in a parallel versus antiparallel manner.

The two strands of DNA are aligned antiparallel to each other in nature. When we say two strands of DNA are aligned antiparallel to each other that means if one strand is in 5' to 3' direction other would be in 3' to 5' direction. This allows nucleotides of two strands to develop attractive forces among them and provide strength to molecule of DNA. Such as there are three hydrogen bonds present between guanine and cytosine while two hydrogen bonds are present between adenine and thymine of antiparallel complementary DNA strands.

Let us suppose these strands are present parallel to each other than there would be no hydrogen bonding between the nucleotides of parallel strands of DNA. It is because the similar charges of similar nucleotides would face each other in parallel strands so instead of attraction forces they will start to repel each other. So called nature did the best.