a) In nucleic acids the nucleotide sub units linked by hydrogen bond
b) Nucleic acids control heredity at molecular level
c) DNA is a dinucleotide and RNA is oligonucleotide

The correct statement is

In nucleic acids, the nucleotide units linked by phosphate ester linkages. Hence, the statement a is incorrect.
Nucleic acids control heredity at molecular level. DNA is the repository of the heredity information.
Thus, the statement b is correct.
DNA and RNA are polynucleotides. Thus, the statement c is incorrect.