The Max Planck quantum theory was initially regarded as absurd, unnatural, and…

'Incompatible' describes two things that clash or cannot exist together — one is essentially inconsistent with, or opposed to, the other. It differs from words about perception ('discernible') or decoding ('decipherable'), which describe how easily something is noticed or understood, not whether it conflicts with something else.

The sentence lists three qualities the Max Planck quantum theory was accused of: 'absurd', 'unnatural', and ____ 'with common sense'. The phrase 'incompatible with' is a standard collocation meaning 'in conflict with' or 'unable to coexist with' — exactly the sense needed here, since the theory's ideas (like energy existing in discrete quanta) directly clashed with everyday, common-sense intuitions about a continuous physical world.

• 'Consanguineous' means related by blood descent — it describes a family relationship, not a conceptual clash between a theory and common sense.

• 'Discernible' means able to be noticed or perceived — it would suggest the theory was easily recognisable, the opposite of the conflict the sentence describes.

• 'Decipherable' means able to be decoded or figured out — it applies to codes or unclear messages, not to whether an idea agrees with common sense.

Only 'incompatible' both completes the required collocation 'incompatible with' and matches the sense of conflict set up by 'absurd' and 'unnatural', confirming it as the correct answer.