Step 1: Concept
In an open organ pipe, the fundamental frequency corresponds to a standing wave pattern where both ends of the pipe are antinodes. The length of the pipe $L$ is equal to one-half of the wavelength $\lambda$ because there is no node at either end.
Step 2: Meaning
The question asks for the wavelength $\lambda$ of the sound wave produced by an open organ pipe that resonates at its fundamental frequency, given the length $L$ of the pipe.
Step 3: Analysis
For an open organ pipe:
The fundamental mode has a single antinode in the middle and nodes at both ends.
This means the distance between two consecutive nodes (or antinodes) is equal to one wavelength $\lambda$.
Since the length $L$ of the pipe spans from one node to another, it is half of the wavelength.
Therefore:
\[L = \frac{\lambda}{2}\]
Solving for $\lambda$, we get:
\[\lambda = 2L\]
This confirms that option A) $2L$ is the correct answer.
Step 4: Conclusion
The length $L$ of an open organ pipe corresponds to half a wavelength at its fundamental frequency. Thus, the full wavelength is twice this length.
Final Answer: (A)