sin−1x=2sin−1a −2π≤sin−1x≤2π;∴−2π≤2sin−1a≤2π−4π≤sin−1a≤4π or 2−1≤a≤21 ∴∣a∣≤21( As 21>21 ). Out of given four option no one is absolutely correct but (c) could be taken into consideration. →∣a∣≤21 is correct, if a<21 is taken as correct then it domain satisfy for a=31 but equation is satisfied. 21>31>21