The total number of unpaired electrons present in the d^3 , d^4...

In an octahedral complex, the d-orbitals split into $t_{2g}$ and $e_g$ levels.

For $d^3$: Configuration is $t_{2g}^3 e_g^0$. Number of unpaired electrons = $3$.

For $d^4$ (low spin): Configuration is $t_{2g}^4 e_g^0$. Number of unpaired electrons = $2$.

For $d^5$ (high spin): Configuration is $t_{2g}^3 e_g^2$. Number of unpaired electrons = $5$.

For $d^6$ (high spin): Configuration is $t_{2g}^4 e_g^2$. Number of unpaired electrons = $4$.

For $d^7$ (low spin): Configuration is $t_{2g}^6 e_g^1$. Number of unpaired electrons = $1$.

Total number of unpaired electrons = $3 + 2 + 5 + 4 + 1 = 15$.

Answer: $15$