What might block the openings of discontinuities if the test specimen surface is not properly cleaned?

The option that identifies potential blockages to openings of discontinuities when a test specimen surface is not properly cleaned is the one that encompasses substances that are commonly found as contaminants, particularly in industrial settings. Materials like paint, scale, and core material are particularly relevant because they can adhere tightly to surfaces, creating a barrier that prevents effective inspection and testing of the substrate underneath.

Paint can accumulate on surfaces and obscure critical discontinuities such as cracks or voids. Scale, which often forms from corrosion or from deposits in metals, can cover up these imperfections as well. Core material refers to residual material from manufacturing processes, potentially obstructing the view of structural integrity issues.

In contrast, other choices mention substances that, while they can create surface contamination, may not be as significant in industrial contexts focused on assessing structural integrity. For instance, water can evaporate, dirt can be easily brushed away, and air does not block discontinuities, while oil, dust, residue, varnish, grease, and plastic do not represent the same level of concern in terms of significant testing obstructions as the materials listed in the correct choice.