Dear Meet
Triangle OAB has the property that angle O = 30° and AB = 1
From sine law
(sin A)/OB = (sin O)/AB
(sin A)/OB = (1/2)/1
OB = 2 sinA
Mazimum of sinA is 1 when A = 90°>
Hence maximum of OB = 2
Option D is correct.
Regards
Arun (askIITians forum expert)