The maximum number of unpaired electrons in a system is determined by the electronic configuration of the central metal ion and the geometry of the complex. Let's analyze each option to determine which one has the maximum number of unpaired electrons.

a) d6 (tetrahedral, high spin):
In a tetrahedral complex, the ligands are arranged around the central metal ion in a tetrahedral shape. The d6 electronic configuration is typically written as (t2g)4(eg)2, where the t2g orbitals (dxy, dxz, dyz) have lower energy and the eg orbitals (dx2-y2, dz2) have higher energy. In a high spin configuration, the electrons prefer to occupy the higher energy eg orbitals before pairing up. Therefore, in this case, there are two unpaired electrons in the eg orbitals.

b) d9 (octahedral):
In an octahedral complex, the ligands are arranged around the central metal ion in an octahedral shape. The d9 electronic configuration is typically written as (t2g)6(eg)3. In this case, all the d orbitals are fully occupied, and there are no unpaired electrons.

c) d4 (octahedral, low spin):
In an octahedral complex, the d4 electronic configuration is typically written as (t2g)3(eg)1. In a low spin configuration, the electrons prefer to occupy the lower energy t2g orbitals before pairing up. Therefore, in this case, there is one unpaired electron in the t2g orbitals.

d) d7 (octahedral, high spin):
In an octahedral complex, the d7 electronic configuration is typically written as (t2g)5(eg)2. In this case, there are two unpaired electrons in the t2g orbitals.

From the analysis above, we can see that option a) d6 (tetrahedral, high spin) has the maximum number of unpaired electrons, which is two.

Therefore, the correct answer is option 'A' - d6 (tetrahedral, high spin).