**Core Concept**
The basal body temperature method is a fertility awareness-based method of family planning that involves tracking a woman's basal body temperature (BBT) to determine ovulation. This method is based on the principle that ovulation is accompanied by a rise in BBT, which remains elevated until the next menstrual period. The "unsafe period" refers to the time when conception is most likely to occur.

**Why the Correct Answer is Right**
The basal body temperature typically remains elevated for 3-4 days after ovulation, during which time the likelihood of conception is highest. This is because sperm can survive inside the female reproductive tract for up to 5 days, and the egg is viable for 24 hours after ovulation. Therefore, the unsafe period lasts for 72 hours, which is the time when the chances of conception are highest. This is because the sperm can fertilize the egg during this period, and the resulting zygote can implant in the uterus.

**Why Each Wrong Option is Incorrect**
**Option A:** 24 hours is too short, as the sperm can survive for up to 5 days and the egg is viable for 24 hours after ovulation.
**Option B:** 48 hours is still too short, as the sperm can fertilize the egg during the first 72 hours after ovulation.
**Option D:** 120 hours is too long, as the likelihood of conception decreases after the first 72 hours.

**Clinical Pearl / High-Yield Fact**
The basal body temperature method is not 100% effective and should be used in conjunction with other methods of family planning, such as barrier methods or hormonal contraception. It is essential to note that the accuracy of this method can be improved by combining it with other fertility awareness-based methods, such as cervical mucus observation and ovulation predictor kits.

**✓ Correct Answer: C. 72 hours**