Given that the length of segment \( \overline{AB} \) is 22.3 millimeters, we need to determine the measure of \( \overline{AC} \).
From the description, it seems that point \( C \) is created by constructing a copy of segment \( \overline{AB} \) starting at point \( B \). This means that \( C \) is actually located at a distance equal to \( \overline{AB} \) from point \( B \). Therefore, we can express the length of \( \overline{AC} \) as follows:
\[
\overline{AC} = \overline{AB} + \overline{BC}
\]
Given that \( \overline{BC} \) is another copy of \( \overline{AB} \), we can replace \( \overline{BC} \) with \( \overline{AB} \):
\[
\overline{BC} = \overline{AB} = 22.3 \text{ mm}
\]
Thus, the total length becomes:
\[
\overline{AC} = \overline{AB} + \overline{AB} = 22.3 \text{ mm} + 22.3 \text{ mm} = 44.6 \text{ mm}
\]
Therefore, the correct measure of \( \overline{AC} \) is **44.6 millimeters**.
Thus, the correct response is:
**The measure of \( \overline{AC} \) is 44.6 millimeters.**