Both lithium and magnesium display several similar properties due to the diagonal relationship; however, the one which is incorrect is:

We begin by recalling that lithium (Li) is placed in Group 1 while magnesium (Mg) is in Group 2 of the periodic table, yet because they occupy diagonally adjacent positions (Li in period 2, Mg in period 3) they often exhibit what is called the diagonal relationship. This relationship causes many of their properties to resemble each other more closely than they resemble the other members of their respective groups.

To decide which of the four given statements is not a correct similarity, we shall examine every option one by one, writing out the relevant chemical facts and, wherever necessary, simple equations.

Option A — Both form soluble bicarbonates

The bicarbonate of lithium is $$LiHCO_3$$ and that of magnesium is $$Mg(HCO_3)_2$$. Both of these bicarbonates exist in aqueous solution and remain soluble. Chemically we may write their presence in water as

$$Li_2CO_3 + CO_2 + H_2O \rightarrow 2\,LiHCO_3$$

$$MgCO_3 + CO_2 + H_2O \rightarrow Mg(HCO_3)_2$$

Because both bicarbonates are indeed soluble, statement A is true and therefore a correct similarity.

Option B — Both form nitrides

When heated in nitrogen, each metal reacts to give a nitride:

$$6\,Li + N_2 \rightarrow 2\,Li_3N$$

$$3\,Mg + N_2 \rightarrow Mg_3N_2$$

Thus both lithium and magnesium form stable nitrides, confirming that statement B is also true.

Option C — Nitrates of both Li and Mg yield $$NO_2$$ and $$O_2$$ on heating

The thermal decomposition of lithium nitrate proceeds as

$$4\,LiNO_3 \rightarrow 2\,Li_2O + 4\,NO_2 + O_2$$

Magnesium nitrate behaves in an almost identical manner:

$$2\,Mg(NO_3)_2 \rightarrow 2\,MgO + 4\,NO_2 + O_2$$

In both cases the products include $$NO_2$$ and $$O_2$$, so statement C is again true.

Option D — Both form basic carbonate

First, let us understand what the phrase basic carbonate means. A basic carbonate is a salt that contains both carbonate $$CO_3^{2-}$$ and hydroxide $$OH^-$$ ions within the same solid, for example

$$MgCO_3 \cdot Mg(OH)_2 \cdot 5H_2O$$

This substance is commonly called basic magnesium carbonate. Magnesium definitely forms such a compound.

Now we check lithium. The only stable carbonate of lithium is the normal salt

$$Li_2CO_3$$

This carbonate does not contain any hydroxide ion in its lattice; nor is any solid compound of the type $$Li_2CO_3 \cdot LiOH \cdot xH_2O$$ known under ordinary conditions. Therefore lithium does not form a basic carbonate.

Because the property is present for magnesium but absent for lithium, statement D is false. Hence it is the single incorrect similarity in the list.

After examining all four options we have found that only Option D fails to describe a genuine similarity arising from the diagonal relationship of Li and Mg.

Hence, the correct answer is Option D.