Pores are:

• A. Internal voids in a material.
• B. A dimensional change under load.
• C. The measure of the acidity of a solution.
• D. The regular lattice arrangement of atoms.

Answer: (A)

Pores are internal voids in a material. They are empty spaces inside the solid that may be connected to the surface or completely enclosed. Their presence lowers density, can create pathways for fluids or gases, and influences diffusion, strength, and fracture behavior. In practice, porosity helps engineers assess integrity and potential leakage or corrosion, which is especially important for materials used in nuclear contexts where irradiation can generate gas-filled pores.

The other ideas don’t fit because dimensional change under load describes deformation or strain, not voids. The measure of acidity of a solution is pH, unrelated to internal voids. And a regular lattice arrangement of atoms refers to the crystal lattice, not to void spaces within the material.