**Core Concept**
Active surveillance is a method of disease surveillance that involves actively seeking out and collecting data on all cases of a disease within a population, rather than relying on passive reporting by healthcare providers or individuals. This approach allows for a more accurate estimate of the true morbidity in a population.

**Why the Correct Answer is Right**
Active surveillance is the most effective method for calculating true morbidity because it involves a systematic and comprehensive search for all cases of a disease, regardless of whether they are reported or not. This approach helps to identify cases that may have gone unreported, such as mild or asymptomatic cases, which are often missed in passive surveillance systems. Active surveillance typically involves a combination of data collection methods, including medical record reviews, laboratory testing, and community outreach, to ensure that all cases are identified and counted.

**Why Each Wrong Option is Incorrect**
**Option A:** Sentinel surveillance is a method of surveillance that involves monitoring a small, representative sample of the population for disease outbreaks or trends. While sentinel surveillance can provide valuable information, it is not a comprehensive method for calculating true morbidity.

**Option B:** Passive surveillance relies on voluntary reporting by healthcare providers or individuals, which can lead to underreporting of cases and an inaccurate estimate of morbidity.

**Option D:** Monitoring is a general term that refers to the process of tracking and analyzing data over time. While monitoring is an important aspect of disease surveillance, it is not a specific method for calculating true morbidity.

**Clinical Pearl / High-Yield Fact**
To accurately calculate true morbidity, active surveillance must be systematic, comprehensive, and unbiased. This requires careful planning, data collection, and analysis to ensure that all cases are identified and counted.

**✓ Correct Answer: C. Active Surveillance**