Step 1: Understand the concept
De Broglie explained Bohr's quantisation by saying that the electron's standing wave must fit the orbit: the circumference of the orbit equals a whole number of wavelengths.
Step 2: Write the condition
\[ 2\pi r = n\lambda \]
Step 3: Solve
\[ \lambda = \frac{2\pi r}{n} \]
Step 4: Result
Option (C). This is equivalent to Bohr's angular momentum condition \(mvr = \dfrac{nh}{2\pi}\) with \(\lambda = \dfrac{h}{mv}\).
Final Answer:
The wavelength is 2 pi r / n. This is option (C).
\[ \boxed{\text{(C) }\frac{2\pi r}{n}} \]