**Core Concept**
The completed family size is a measure of total fertility, which reflects the average number of children a woman would have in her lifetime, assuming a certain level of fertility and mortality rates. It is an essential indicator of reproductive health and population dynamics.

**Why the Correct Answer is Right**
The completed family size is typically estimated using the Gross Reproduction Rate (GRR) and Net Reproduction Rate (NRR). The GRR is the average number of daughters a woman would have in her lifetime, while the NRR takes into account mortality rates. The completed family size is often estimated to be around 2.1, considering the average number of children a woman would have, along with the replacement rate for the next generation. This is because it takes, on average, 2.1 children to replace the parents and maintain a stable population.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not account for mortality rates, which are crucial in determining the completed family size.
**Option B:** This option is incorrect because it represents the average number of children a woman would have, but it does not consider the replacement rate for the next generation.
**Option C:** This option is incorrect because it is too low, not taking into account the average number of children a woman would have in her lifetime.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a completed family size of 2.1 is often used as a benchmark to determine the replacement rate for the next generation, considering mortality rates and the average number of children a woman would have in her lifetime.

**Correct Answer: D. 2.1**