B

Step 1: Understand luminous efficiency. Luminous efficiency is a measure of how well a light source converts electrical energy into visible light, usually expressed in lumens per watt (lm/W).
Step 2: Compare the two light sources. Traditional incandescent bulbs (like a 40 W bulb) typically have a luminous efficiency of about 10-17 lm/W, while fluorescent tubes usually achieve around 35-100 lm/W.
Step 3: In this case, a standard 40 W fluorescent tube will have a significantly higher luminous efficiency compared to a 40 W incandescent bulb.
Conclusion: Since the fluorescent tube is generally more efficient at converting energy into light, the correct answer is Option B: A 40 W fluorescent tube.