Recall the definition of adenosine

Adenosine is a nucleoside formed by:

• the nitrogenous base adenine, and
• the sugar $\beta$-D-ribose.

So, structurally, adenosine is 9-$\beta$-D-ribofuranosyl adenine.

Key structural features of adenosine

A correct structure of adenosine must contain:

1. Adenine base = a purine ring with an $-NH_2$ group at the 6-position.
2. Ribose sugar = a five-membered furanose ring.
3. A $\beta$-N-glycosidic bond between:
   • N-9 of adenine and
   • C-1' of ribose.
4. Since it is adenosine and not deoxyadenosine, the sugar must have $-OH$ groups at both 2' and 3' positions.

How to distinguish from similar biomolecules

• If the sugar lacks the 2'-OH, it would be deoxyadenosine, not adenosine.
• If a phosphate group is present, it would be AMP/ADP/ATP, not adenosine.
• If the base is not adenine, then it is another nucleoside.

Match with the given options

Although the actual drawings for options A–D are not visible here, the stored correct answer is A, which implies that Option A is the structure showing adenine attached to $\beta$-D-ribose through N-9 to C-1' glycosidic linkage, i.e. adenosine.

Final conclusion

Therefore, the correct structure of adenosine is Option A.