**Core Concept**
The partogram is a graphical representation used to monitor the progress of labor in a pregnant woman. It is essential for obstetricians to accurately document the initial markings on the partogram to assess the progression of labor and make timely decisions.

**Why the Correct Answer is Right**
The initial markings on the partogram are made on the "ale line" (also known as the Ferguson line or the 10 cm line), which represents the expected duration of the second stage of labor. This line is used as a reference point to assess the progress of the second stage of labor and to identify any deviations from the expected pattern. The initial markings include the time of the onset of the second stage of labor and the time when the cervix is 10 cm dilated.

**Why Each Wrong Option is Incorrect**
**Option A:** The left side of the action line is incorrect because the initial markings are not made on this line, which represents the expected duration of the first stage of labor.
**Option B:** The action line is incorrect because it represents the expected duration of the first stage of labor, not the second stage.
**Option D:** The right side of the ale line is incorrect because the initial markings are made on the ale line itself, not on the right side of it.

**Clinical Pearl / High-Yield Fact**
It is essential to accurately document the initial markings on the partogram to avoid errors in assessing the progress of labor and to ensure timely interventions.

**✓ Correct Answer: C. On the ale line**