If y=21logx4, then
y=21logx4
Taking log on both sides with base 2
⇒log2y=log221logx4
{∵ak=x;x≠1logax=k;x>0}
⇒log2y=1logx4
[logab=1logba]
⇒log2y=log4x
⇒log2y=log22x
⇒log2y=12log2x
[logakx=1klogax]
⇒log2y=log2x12
⇒y=x12
Hence the correct answer is Option C.