The backbone of a polynucleotide chain is formed by:

The correct answer is Option (3) → Sugar and phosphates

Option (3) Sugar and phosphates — Correct. In a polynucleotide chain, nucleotides are joined by phosphodiester bonds between the sugar of one and the phosphate of the next. So sugar and phosphates form the backbone, and the bases project from it.

NCERT Extract:
Structure of Polynucleotide Chain – “… A polymer thus formed has at one end a free phosphate moiety at 5’ -end of sugar, which is referred to as 5’-end of polynucleotide chain. Similarly, at the other end of the polymer the sugar has a free OH of 3’ C group which is referred to as 3’ - end of the polynucleotide chain. The backbone of a polynucleotide chain is formed due to sugar and phosphates. The nitrogenous bases linked to sugar moiety project from the backbone (Figure 5.1).”

Figure 5.1 A Polynucleotide chain