**Core Concept**
Direct standardization is a statistical method used to compare the mortality rates between two populations with different age structures. It involves calculating the expected number of deaths in a population if it had the same age distribution as a standard population. This method is necessary to avoid biases introduced by differences in age structures between the two populations.

**Why the Correct Answer is Right**
Direct standardization is essential when comparing mortality rates between populations with different age structures because it allows for a fair comparison by removing the confounding effect of age. By using a standard population as a reference, direct standardization enables researchers to calculate the expected number of deaths in each population if they had the same age distribution as the standard population. This is particularly important when comparing countries with different age structures, as it helps to identify true differences in mortality rates rather than those due to age.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because indirect standardization is used to compare mortality rates between populations with different age structures, but it does not remove the confounding effect of age.
**Option B:** is incorrect because it is not a parameter that makes direct standardization necessary.
**Option C:** is incorrect because it is a characteristic of the standard population, not a parameter that makes direct standardization necessary.

**Clinical Pearl / High-Yield Fact**
When comparing mortality rates between populations, direct standardization is essential to avoid biases introduced by differences in age structures. This can be remembered by the mnemonic "DSS": Direct Standardization to avoid Statistical biases.

**Correct Answer:** C.