Question

Which of the following is the simplest unit of a nucleic acid

Answer 1

Its nucleotides...its contains sugar(deoxyribose in DNA and ribose in RNA) phosphate and a nitrogen base(A,C,G,T)

Answer 2

The simplest unit of a nucleic acids is known as nucleotides. 
A nucleotide consist of three distinct chemical groups
    - a 5-carbon sugar(ribose and deoxyribose)
    -a nitrogen-rich base(cytosine (C), guanine (G), adenine (A), thymine (T) in      DNA or uracil (U) instead of T (in RNA)
    -phosphate