The haemoglobin content per 100 ml of blood in a normal healthy human adult is actually found in the range of 12 - 16 grams. Therefore, the correct answer is Option D: 12 - 16 mg. However, it's important to note that there seems to be a mistake in the unit provided in the options. The correct unit should be grams (g) and not milligrams (mg), as haemoglobin content in blood is measured in grams per deciliter (g/dL). So, the correct expression reflecting normal haemoglobin levels should be 12 - 16 g/dL, not mg.

In general, average ranges for haemoglobin levels vary by age and sex, typically:

• For men: 13.8 to 17.2 grams per deciliter
• For women: 12.1 to 15.1 grams per deciliter

These levels can be affected by various factors including diet, physical activity, and medical conditions. It's essential for individuals to have their haemoglobin levels within the normal range as it's crucial for effectively transporting oxygen throughout the body. Deviations from the normal range can indicate different medical conditions that may require further investigation.