Who developed the modern model of the solar nebular disk instability model?

George Wetherill and Victor Safronov

Explanation:

The modern model known as "solar nebular disk instability model" was developed by George Wetherill after refining the basic modern model developed by  Russian astronomer Victor Safronov in 1970.  According to this model, dust and gases resulted into massive clumps during the early stage of formation of solar system. This model well explains the formation of star, sun, solar system, planet, asteroids and comets.

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it is either c or d and i know for sure it is not a or b