Ans will be O(n/log n)
if we check for height ,
when, n=3, height=3/log3=3/1.58=3/2=1
n=6, height=6/log6=6/2.58=6/3=2
n=10, height=10/log10=10/3.32=10/3=3.33=3
n=15, height=15/log15=15/3.90=15/4=3.75=4
n=21 height=21/log21=21/4.39=21/4=5
n=28 height=28/log28=28/4.8=28/5=5.6=6
so if we further expand the tree only (d) will give correct answer