The correct options are
B cos−1∣∣∣1√3∣∣∣
D tan−1√2
→a,→b,→c are vectors of equal magnitude and mutually perpendicular to each other then angle between →a+→b+→c & →a is given by
cosθ=(→a+→b+→c).→a∣∣∣→a+→b+→c∣∣∣∣∣→a∣∣=(→a+→b+→c).→a√→a2+→b2+→c2∣∣→a∣∣=∣∣→a∣∣√→a2+→b2+→c2=1√3
⇒θ=cos−1(1√3)=tan1√2
Hence, options B and D.