Identify the trend in acidity of group 16 hydrides

The hydrides of group 16 are: $\mathrm{H_2O,\ H_2S,\ H_2Se,\ H_2Te}$

Down the group, acidity increases: $\mathrm{H_2O < H_2S < H_2Se < H_2Te}$

Therefore, Assertion A: $\mathrm{H_2Te} \text{ is more acidic than } \mathrm{H_2S}$ is true.

Reason for increase in acidity down the group

As we move down the group, the size of the central atom increases, so the bond between hydrogen and the central atom becomes weaker.

Thus, the bond dissociation enthalpy follows: $\mathrm{H-Te < H-S}$ meaning the $\mathrm{H-Te}$ bond is easier to break than the $\mathrm{H-S}$ bond.

Hence, Reason R: $\text{Bond dissociation enthalpy of } \mathrm{H_2Te} \text{ is lower than } \mathrm{H_2S}$ is also true.

Check whether R explains A

Acidity of these hydrides depends mainly on how easily they can release $\mathrm{H^+}$. A weaker $\mathrm{H-E}$ bond means easier proton release.

Since $\mathrm{H_2Te}$ has lower bond dissociation enthalpy than $\mathrm{H_2S}$, it donates proton more easily and is therefore more acidic.

So, R is the correct explanation of A.

Conclusion

Both Assertion and Reason are true, and Reason correctly explains Assertion.

Therefore, the correct option is: $\boxed{\text{A}}$