In context with the transition elements, which of the following statements is incorrect?

The question asks us to find the incorrect statement about transition (d-block) elements. We shall examine every option one by one.

Option A: “In addition to the normal oxidation states, the zero oxidation state is also shown by these elements in complexes.”
• Many transition metals form neutral (zero-oxidation-state) carbonyls and related complexes, e.g. $$Ni(CO)_4$$, $$Fe(CO)_5$$, $$Cr(CO)_6$$, $$V(CO)_6$$.
• Hence a zero oxidation state is indeed possible for several transition metals.
Therefore, Option A is a correct statement.

Option B: “In the highest oxidation states, the transition metal show basic character and form cationic complexes.”
• At very high oxidation states the metal centre possesses a high positive charge density. This makes the species strongly electrophilic (acidic), not basic.
• Such metals commonly form oxo-anions and covalent oxides, e.g. $$MnO_4^-$$ (Mn in +7), $$Cr_2O_7^{2-}$$ (Cr in +6), $$VO_3^-$$ (V in +5). These are anionic or neutral covalent species, not cationic complexes.
Therefore, Option B is an incorrect statement.

Option C: “In the highest oxidation states of the first five transition elements (Sc to Mn), all the $$4s$$ and $$3d$$ electrons are used for bonding.”
Sc: $$[Ar]\,3d^1 4s^2 \rightarrow +3$$ (uses 3 electrons)
Ti: $$[Ar]\,3d^2 4s^2 \rightarrow +4$$ (4 electrons)
V: $$[Ar]\,3d^3 4s^2 \rightarrow +5$$ (5 electrons)
Cr: $$[Ar]\,3d^5 4s^1 \rightarrow +6$$ (6 electrons)
Mn: $$[Ar]\,3d^5 4s^2 \rightarrow +7$$ (7 electrons)
• In every case the sum of all $$4s$$ and $$3d$$ electrons equals the group number and is employed in the maximum oxidation state. Hence the statement is true.
Therefore, Option C is a correct statement.

Option D: “Once the $$d^5$$ configuration is exceeded, the tendency to involve all the 3d electrons in bonding decreases.”
• Beyond Mn (which is $$d^5$$), the maximum attainable oxidation states begin to drop: $$Fe^{+6}$$ (rare), $$Co^{+5}$$ (rare), $$Ni^{+4}$$, $$Cu^{+3}$$, $$Zn^{+2}$$. Thus, metals with more than five d electrons usually do not utilise all of them in bonding.
Therefore, Option D is a correct statement.

Only Option B conflicts with the well-established behaviour of transition metals in high oxidation states. Hence the incorrect statement is:

Option B which is: In the highest oxidation states, the transition metal show basic character and form cationic complexes.