**Core Concept**
The volume of distribution (Vd) is a pharmacokinetic parameter that represents the apparent volume that a drug would need to occupy to account for its total body concentration. It is calculated using the formula: Vd = Dose / Concentration.

**Why the Correct Answer is Right**
To determine the volume of distribution, we can use the formula Vd = Dose / Concentration. Given that the dose of drug X is 4.0 g and its plasma concentration is 50 mg/ml, we can plug in these values to calculate the Vd. First, we need to convert the dose from grams to milligrams, as the concentration is given in milligrams per milliliter (mg/ml). Since 1 g = 1000 mg, the dose is 4.0 g = 4000 mg. Now, we can calculate the Vd using the formula: Vd = 4000 mg / 50 mg/ml = 80 ml.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we cannot evaluate it.
**Option B:** This option is not provided, so we cannot evaluate it.
**Option C:** This option is not provided, so we cannot evaluate it.
**Option D:** This option is not provided, so we cannot evaluate it.

**Clinical Pearl / High-Yield Fact**
A high volume of distribution indicates that a drug is widely distributed throughout the body, often to peripheral tissues. This can lead to a longer duration of action and a lower peak concentration, as the drug is gradually released from these tissues.

**Correct Answer:** . 80 ml.