**Core Concept**
The elimination of a drug from the body can be affected by renal excretion, hepatic metabolism, and biliary secretion. In this case, the majority of drug X is eliminated through renal excretion (80%), making it a renally cleared drug. The patient's renal function, as indicated by their glomerular filtration rate (GFR), plays a crucial role in determining the appropriate dose.

**Why the Correct Answer is Right**
A 65-year-old patient with a GFR of 60 ml/min has decreased renal function compared to a normal GFR of 120 ml/min. Since drug X is primarily eliminated through the kidneys, a reduced GFR will lead to decreased clearance of the drug. To maintain therapeutic levels, the dose rate needs to be adjusted. The patient's liver and biliary functions are normal, so adjustments are not needed for hepatic metabolism or biliary secretion. The dose rate should be reduced proportionally to the decrease in GFR.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option does not take into account the patient's decreased renal function, which is the primary factor affecting drug X's elimination.
* **Option B:** This option does not provide a clear adjustment for the patient's GFR, which is necessary for a renally cleared drug like drug X.
* **Option C:** This option may be a correct adjustment for a drug with a different elimination profile, but it does not account for the patient's specific renal function and the predominant renal excretion of drug X.

**Clinical Pearl / High-Yield Fact**
When adjusting doses for patients with renal impairment, it's essential to consider the drug's elimination profile, particularly if it's primarily cleared through the kidneys. A reduced GFR often necessitates a decrease in the dose rate to avoid toxicity.

**Correct Answer:** B. 37.5 mg/hour.