## **Core Concept**
The question tests the understanding of sperm physiology, specifically the duration of sperm motility after ejaculation into the vagina. Sperm motility is crucial for fertilization as it enables sperm to move through the female reproductive tract.

## **Why the Correct Answer is Right**
The correct answer, **C. 48 hours**, reflects the generally accepted duration for which sperm can retain their motility after ejaculation into the vagina. This duration allows sperm to survive the passage through the cervix, uterus, and fallopian tubes, increasing the chances of fertilization. The survival and motility of sperm are influenced by the environment of the female reproductive tract.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Less than 30 minutes* - This is incorrect because sperm can survive much longer than 30 minutes in the vagina.
- **Option B:** *24 hours* - While sperm can survive for up to 5 days in some cases within the female reproductive tract, 24 hours underestimates the generally accepted duration for motility.
- **Option D:** *5 days* - This option overestimates the duration for which sperm retain motility; while sperm can survive in the female tract for up to 5 days, their motility significantly decreases after 48 hours.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while sperm can survive inside the female reproductive tract for up to 5 days, their ability to fertilize an egg is highest within the first 48 hours after ejaculation. Factors such as cervical mucus quality, timing of ovulation, and sperm count can affect fertility.

## **Correct Answer:** . 48 hours