Check statement A

Transition elements can show zero oxidation state in many complexes, especially with ligands like CO.

Example: $\mathrm{Ni(CO)_4},\ \mathrm{Fe(CO)_5},\ \mathrm{Cr(CO)_6}$

So, A is correct.

Check statement B

In their highest oxidation states, transition metals generally show acidic or sometimes amphoteric character, not basic character.

Examples:

• $\mathrm{V_2O_5}$ is acidic
• $\mathrm{CrO_3}$ is acidic
• $\mathrm{Mn_2O_7}$ is acidic

Also, in high oxidation states, they usually form oxoanions or covalent compounds rather than simple cationic complexes.

Therefore, the statement that they "show basic character and form cationic complexes" is incorrect.

Check statement C

For the first five transition elements, Sc to Mn, the highest oxidation state corresponds to use of all available $4s$ and $3d$ electrons in bonding:

• Sc: $+3$
• Ti: $+4$
• V: $+5$
• Cr: $+6$
• Mn: $+7$

This matches the statement. So, C is correct.

Check statement D

After $d^5$ configuration, pairing increases and the tendency to involve all $3d$ electrons in bonding decreases.

Hence, elements after Mn generally do not show oxidation states as high as the total number of $4s + 3d$ electrons.

So, D is correct.

Conclusion

The incorrect statement is:

$\boxed{\text{B}}$

Comparison with stored answer

Stored correct answer = B

My derived answer = B

So, they agree.