Identify the reaction being referred to

The options clearly describe nitrosation of an aromatic amine under acidic conditions at low temperature.

In such reactions, the active electrophile is the nitrosonium ion: $\mathrm{NO^+}$

This is usually generated from nitrous acid in acid medium: $\mathrm{NaNO_2 + HCl \rightarrow HNO_2}$ $\mathrm{HNO_2 + H^+ \rightarrow NO^+ + H_2O}$

What happens with aromatic amines?

For strongly activated aromatic rings such as aniline or substituted anilines, nitrosation occurs on the ring, mainly at the para-position (if available), especially at low temperature.

Thus the product $\mathrm{B}$ is typically a $p$-nitroso compound.

Check each option

Option A: $'$\mathrm{B}$' is N-nitroso ammonium compound$

This is incorrect.

In the given aromatic nitrosation, the main product is not an $N$-nitroso ammonium compound; rather, it is a ring-nitrosated product, generally the $p$-nitroso derivative.

So option A is false.

Option B: $The product '$\mathrm{B}$' formed in the above reaction is$p$-nitroso compound at low temperature$

This is correct.

Aromatic amines under nitrosation conditions at low temperature often give para-nitroso substitution on the ring.

Option C: $The electrophile involved in the reaction is$\mathrm{NO^+}$$

This is correct.

The nitrosonium ion $\mathrm{NO^+}$ is the attacking electrophile in nitrosation.

Option D: The reaction occurs at low temperature

This is correct.

Nitrosation reactions of this type are typically carried out at low temperature to control the reaction and stabilize intermediates/products.

Conclusion

The incorrect statement is: $\boxed{\text{A}}$