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Laser beams are used to measure long distances because:

$\begin {array} {1 1} (a)\;They\: are\: monochromatic & \quad (b)\;They \: are\: polarized \\ (c)\;They \: are \: coherent & \quad (d)\;They\: have\: high\: degree\: of\: parallelism \end {array}$

Laser beams are perfectly parallel. So that they are very narrow and can travel a long distance without spreading.
Ans : (d)