**Core Concept**
The Motor Vehicle Act of 1988 in India specifies a permissible blood alcohol level for driving vehicles. This limit is crucial for determining whether a driver is considered intoxicated and liable for penalties under the law.

**Why the Correct Answer is Right**
The permissible blood alcohol level is 30 milligrams per 100 milliliters (mg/100ml) for drivers, as per the Motor Vehicle Act. This limit is set to ensure that drivers are not impaired by alcohol while operating vehicles. The law assumes that a driver with a blood alcohol level below this threshold is unlikely to be intoxicated and thus can continue driving safely.

**Why Each Wrong Option is Incorrect**
**Option A:** 20 mg/100ml - This is lower than the actual permissible limit, making it an incorrect option.
**Option B:** 50 mg/100ml - This is higher than the permissible limit, making it an incorrect option.
**Option C:** 40 mg/100ml - Although close, this is still not the exact permissible limit specified by the Motor Vehicle Act.

**Clinical Pearl / High-Yield Fact**
It is essential for medical professionals to be aware of the permissible blood alcohol level as per the Motor Vehicle Act, as they may be required to provide medical certificates or testify in court cases related to driving under the influence.

**Correct Answer: D. 30 mg/100ml**