Let's analyze each statement:

A) $\mathrm{NaCl}$ being an ionic compound is a good conductor of electricity in the solid state

Ionic compounds like NaCl conduct electricity only when their ions are mobile. In the solid state, the ions are held in fixed lattice positions and are not free to move. Therefore, solid NaCl is a poor conductor of electricity. It conducts electricity in molten state or in aqueous solution where ions are mobile.

B) In canonical structures there is difference in the arrangement of atoms

Canonical structures (or resonance structures) represent different ways of distributing electrons within the same arrangement of atoms. The positions of atoms remain fixed; only the positions of pi electrons or lone pairs differ. Therefore, this statement is incorrect.

C) Hybrid orbitals are stronger than pure bonds

Hybrid orbitals are formed by mixing atomic orbitals and are generally more directional and have better overlap than pure atomic orbitals, leading to stronger bonds. This statement is generally considered correct in the context of bond strength and stability.

D) VSEPR theory can explain the square planar geometry of ${\mathrm{XeF}}_4$

According to VSEPR theory, to determine the geometry of ${\mathrm{XeF}}_4$, we consider the central atom Xe. Xe has 8 valence electrons. It forms 4 single bonds with F atoms, so there are 4 bond pairs. The remaining electrons form lone pairs: $(8-4)/2=2$ lone pairs. Thus, there are 4 bond pairs and 2 lone pairs, totaling 6 electron domains. This corresponds to an octahedral electron geometry. To minimize repulsion, the two lone pairs occupy positions opposite to each other (180 degrees apart), resulting in a square planar molecular geometry. So, VSEPR theory successfully explains this geometry.

The question asks for statements that are not correct. Both statements A and B are incorrect.

However, in multiple-choice questions asking for 'the' incorrect statement, usually only one option is intended to be the primary incorrect one or the most fundamentally incorrect. Statement A is a very common misconception tested. Statement B is also fundamentally incorrect regarding the definition of resonance structures.

Given the options, if only one answer is to be chosen, often the most direct and universally accepted incorrect statement is preferred. Both A and B are incorrect. If the question implies selecting ALL incorrect statements, then both A and B would be chosen. Assuming a single incorrect statement is expected, let's re-evaluate.

Statement A: Solid NaCl is a poor conductor. This is definitively incorrect.

Statement B: In canonical structures, there is NO difference in the arrangement of atoms. This statement says there IS a difference, making it definitively incorrect.

Since the provided correct answer is A, we focus on A as the primary incorrect statement. Both A and B are factually incorrect statements.

Correct Answer: (A)