Which of the following system has a maximumn number of the unpaired electrons in an inner octahedral complex?
 

Strong field ligands are strong enough to pair up the electrons. 

Inner d-orbital complex means a strong field ligand is present. 
For ${\mathrm{d}}^9$, 6 electrons paired up in three orbitals in ${\mathrm{t}}_{2\mathrm{g}}$ and three electrons in ${\mathrm{e}}_{\mathrm{g}}$. In  ${\mathrm{e}}_{\mathrm{g}}$ , 2 electrons are paired up with one unpaired electron.  
For ${\mathrm{d}}^7$, 6 electrons paired up in three orbitals in ${\mathrm{t}}_{2\mathrm{g}}$ and one unpaired electron in eg. 
For ${\mathrm{d}}^5$ ,5 electrons in ${\mathrm{t}}_{2\mathrm{g}}$, four electrons paired up with one unpaired electron. 
For ${\mathrm{d}}^4$, 4 electrons out of which two electrons paired up with two unpaired electrons in ${\mathrm{t}}_{2\mathrm{g}}$. 

Hence the correct option is ${\mathrm{d}}^4$.