**Core Concept**
Algor mortis, the cooling of the body after death, is an important factor in estimating the post-mortem interval (PMI). The rate of cooling depends on various factors, including the ambient temperature, the body's initial temperature, and its size.

**Why the Correct Answer is Right**
In a body recovered from an open field, the ambient temperature is likely to be around 20-30°C. After death, the body's temperature drops at a rate of approximately 1.5°C per hour for the first 12-15 hours, and then gradually slows down to about 0.5°C per hour. Given that the body's initial temperature is around 37°C, it would take around 3-5 days for the body to cool down to a temperature close to the ambient temperature. This is because the body's temperature would have dropped to around 25-28°C in 3-5 days, which is consistent with the ambient temperature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the body's temperature would not have cooled down significantly in 24-36 hours, and the ambient temperature would still be higher than the body's temperature.

**Option C:** This option is incorrect because the body's temperature would have cooled down further than 25-28°C in 5-10 days, and it would not be consistent with the ambient temperature.

**Option D:** This option is incorrect because the body's temperature would not remain at a constant level for more than 2 weeks, and it would continue to cool down gradually.

**Clinical Pearl / High-Yield Fact**
The temperature of a body can be used to estimate the post-mortem interval, but it should be used in conjunction with other signs such as rigor mortis, livor mortis, and decomposition to obtain a more accurate estimate.

**✓ Correct Answer: B. 3- 5 days**