Step 1: Understanding the Concept
The efficiency of a Carnot engine depends only on the temperatures of the hot source and the cold sink, both in kelvin.
Step 2: Key Formula or Approach
\[ \eta=1-\frac{T_2}{T_1} \]
Step 3: Substitute
\[ \eta=1-\frac{300}{500}=1-0.6=0.4 \]
\[ \eta=40\% \]
Step 4: Check the options
75% would need the sink at 125 K. 50% needs a sink at 250 K. 25% needs a sink at 375 K. Only 40% matches 300 K, option (D).
Final Answer:
The efficiency is 1 - 300/500 = 40 percent, option (D).
\[ \boxed{40\%} \]