**Core Concept**
The standard error (SE) is a statistical measure that estimates the variability of sample means from a population mean. In this context, it is used to quantify the uncertainty of the mean hemoglobin (Hb) level in a sample of pregnant women.

**Why the Correct Answer is Right**
To calculate the standard error, we use the formula: SE = σ / √n, where σ is the standard deviation of the population (1 gm% in this case) and n is the sample size (100 women). Plugging in the values, we get SE = 1 / √100 = 1 / 10 = 0.1 gm%. This is the standard error of the mean Hb level in this population.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid answer, as it does not represent a numerical value.
**Option B:** This option is not relevant to the calculation of standard error.
**Option C:** This option is not a correct representation of the formula for standard error.

**Clinical Pearl / High-Yield Fact**
When working with sample means, it's essential to consider the standard error to understand the precision of your estimates. A smaller standard error indicates a more reliable estimate of the population mean.

**Correct Answer:** 0.1. The standard error is 0.1 gm%.