$\frac{\log_2\left(\log_2\left(\log_2\left(256\right)\right)\right)}{\log_2\left(9\right)}=\frac{\log_2\left(\log_2\left(8\right)\right)}{\log_2\left(9\right)}=\frac{\log_2\left(3\right)}{\log_2\left(9\right)}=\log_9\left(3\right)=0,5$