**Core Concept**
Good diabetic control is assessed by measuring glycosylated haemoglobin (HbA1c), which reflects the average blood glucose levels over the past 2-3 months. HbA1c is a product of non-enzymatic glycosylation between glucose and the N-terminal valine of the beta-chain of haemoglobin.

**Why the Correct Answer is Right**
The American Diabetes Association (ADA) recommends that good diabetic control is achieved when HbA1c levels are below 7%. This is because higher levels of HbA1c are associated with an increased risk of diabetic complications, such as microvascular disease and macrovascular disease. The normal range for HbA1c is generally considered to be below 6%, but the ADA recommends a target of less than 7% for most adults with diabetes.

**Why Each Wrong Option is Incorrect**
**Option A:** HbA1c levels above 8% indicate poor diabetic control and an increased risk of complications. While HbA1c levels above 7% may indicate some degree of hyperglycemia, levels above 8% are generally considered to be associated with more significant glycemic control issues.
**Option B:** HbA1c levels below 5% may indicate a low-carbohydrate diet or other non-diabetic conditions. However, HbA1c levels below 5% are generally not considered to be a reliable indicator of good diabetic control in people with diabetes.
**Option C:** HbA1c levels are not directly related to the duration of diabetes. While longer duration of diabetes may be associated with higher HbA1c levels, HbA1c levels can be elevated in people with newly diagnosed diabetes.

**Clinical Pearl / High-Yield Fact**
It is essential to note that HbA1c levels can be artificially elevated in people with certain conditions, such as hemoglobinopathies or blood transfusions. Therefore, HbA1c levels should be interpreted in the context of the individual's clinical presentation and laboratory results.

**Correct Answer: D. Below 7%**