Step 1: Understand an open pipe.
An open organ pipe is open at both ends. At each open end the air can move freely, so both ends are antinodes, the points of largest motion.
Step 2: Picture the fundamental mode.
The simplest standing wave has an antinode at each end and one node in the middle. This is the lowest possible note, called the fundamental.
Step 3: Link length to wavelength.
The distance from one antinode to the next antinode is half a wavelength. Since the pipe stretches from one end antinode to the other, the length equals half a wavelength. \[ L = \frac{\lambda}{2} \]
Step 4: Solve for the wavelength.
Multiply both sides by two. \[ \lambda = 2L \]
Step 5: Check the choices.
A value of $4L$ would be for a pipe closed at one end, not open at both. So that does not apply here.
Step 6: State the answer.
The wavelength of the fundamental in an open pipe is twice its length. \[ \boxed{\lambda = 2L} \]