**Core Concept:**
The Pearl Index (PI) is a measure of contraceptive effectiveness that takes into account the proportion of women who become pregnant despite using a contraceptive method. It is calculated by dividing the number of pregnancies by the number of effective woman-months of use and multiplying by 100.

**Why the Correct Answer is Right:**
In this scenario, we are calculating the Pearl Index for oral contraceptive pills (OCPs) over a 20-month follow-up period. Among the 100 women using OCPs, 5 became pregnant. So, we have:

Effective woman-months of use = Total woman-months of use - (Pregnant women × Total woman-months of use)
= (100 × 20 months) - (5 × 20 months)
= 2000 - 100
= 1900 woman-months

Pregnancies = 5

Now, we can calculate the Pearl Index:

PI = (Number of pregnancies) / (Effective woman-months of use) × 100
= (5) / (1900) × 100
= 0.026 × 100
= 2.6

So, the Pearl Index for OCPs in this scenario is 2.6.

**Why Each Wrong Option is Incorrect:**
A. This is the same as the correct answer, so it is incorrect.
B. Similarly, this is the same as the correct answer, so it is incorrect.
C. This is the same as the correct answer, so it is incorrect.

**Clinical Pearl:**
The Pearl Index is a useful tool for evaluating the contraceptive effectiveness of a method. In this context, it indicates that among 100 women using OCPs for 20 months, 2.6 pregnancies would be expected. To improve the contraceptive effectiveness, consider discussing alternative methods like intrauterine devices (IUDs), implants, or barrier methods like condoms.