How many wavelengths of Kr⁸⁶ are there in one metre ?

The correct option is 16,50,763.73.

Historical Context and Definition:
In 1960, the 11th General Conference on Weights and Measures (CGPM) redefined the SI unit of length, the metre, in terms of a natural atomic standard to achieve higher precision and reproducibility than the physical platinum-iridium prototype bar.

Under this definition, the metre was defined based on the wavelength of light emitted by the krypton-86 ($\mathrm{Kr}^{86}$) isotope. Specifically, one metre is equal to exactly 1,650,763.73 wavelengths in vacuum of the orange-red radiation corresponding to the transition between the energy levels ${2p}_{10}$ and ${5d}_5$ of the krypton-86 atom.

Thus, the number of wavelengths of krypton-86 in one metre is 1,650,763.73 (written in Indian numbering system format as 16,50,763.73).