It was proposed that nucleic acids were composed of a single tetramer of four nitrogenous bases linked by a circular, sugar-phosphate backbone. what apparent weakness exists in applying this model of nucleic acid structure to the experimenters' conclusion regarding the source of genetic information?

I think the apparent weakness that exists in applying this model of nucleic acid structures are; If it was single tetramer of 4 then it would not be able to have the complexity of the linear sequence, and also it could account for significant quantity if there were enough individual tetramers. 
DNA molecule is a made up of nucleotides bases, that are linked together in long chains. Each nucleotide is made up of a sugar (deoxyribose sugar), a nitrogenous base, and a phosphate group. The nucleotides bases on one strand of the DNA pairs with the nucleotide on the other strand (adenine pairs with Thyamine and guanine with Cytosine). The secondary structure of the polymer is responsible for the shape that the nucleic acid assumes. 

Green pods should be the a, c, d.