Identify the type of bonds present

The question asks which bond is most susceptible to cleavage in strong acid. In such problems, we usually look for ether/acetal-type C–O bonds that can break after protonation of oxygen.

General rule in acidic cleavage of oxygen-containing compounds

Under strongly acidic conditions:

• Oxygen gets protonated first.
• Then cleavage occurs more readily at the bond which leads to the more stable carbocation (or the bond that is easier to break after protonation).
• In acetals/glycosides, the bond to the anomeric carbon is especially acid-labile.

Examine the labeled bonds

The options are:

• A: $\text{Cl} - O_2$
• B: $O_2 - C_3$
• C: $C_4 - O_5$
• D: $O_5 - C_6$

Among these, the bond most likely to cleave in acid is the C–O bond that, upon protonation, can generate the most stabilized cation/oxocarbenium ion.

Why $O_5 - C_6$ is most susceptible

The $O_5-C_6$ bond is the most acid-sensitive because cleavage there can form a stabilized oxonium/oxocarbenium-type intermediate, which is a common driving force in acid-catalyzed cleavage of such oxygenated organic structures.

In contrast:

• $\text{Cl}-O_2$ is not the preferred bond for simple acid cleavage here.
• $O_2-C_3$ and $C_4-O_5$ are ordinary C–O bonds but are less activated toward cleavage than the bond at the more acid-labile center.

Conclusion

Therefore, the most susceptible site for bond cleavage is:

$\boxed{O_5 - C_6}$

So the correct option is D.