**Core Concept**
The attributable risk, also known as the risk difference, is a measure of the proportion of disease incidence that can be attributed to a specific risk factor. In this case, the attributable risk is used to quantify the increased risk of cervical cancer (Ca. Cx) associated with exposure to multiple sex partners.

**Why the Correct Answer is Right**
The attributable risk is calculated as the difference between the incidence of the disease in the exposed group and the incidence in the unexposed group, divided by the incidence in the exposed group. Given that the woman has 5 times the increased risk of Ca. Cx with multiple sex partners, the attributable risk would be 80%, indicating that 80% of the increased risk of Ca. Cx can be attributed to exposure to multiple sex partners.

**Why Each Wrong Option is Incorrect**
* **Option A:** 20% is incorrect because it underestimates the attributable risk associated with exposure to multiple sex partners.
* **Option B:** 50% is incorrect because it does not accurately reflect the increased risk of Ca. Cx associated with multiple sex partners.

**Clinical Pearl / High-Yield Fact**
The attributable risk is a useful measure for public health policymakers to determine the effectiveness of interventions aimed at reducing the incidence of a disease. In this case, the high attributable risk associated with exposure to multiple sex partners highlights the importance of promoting safe sex practices and cervical cancer screening programs.

**✓ Correct Answer: C. 80%**