what can be observed only by observing its effects on gravity?\no dark matter\no a solar nebula\no gas and…

Brief Explanations:

Dark matter does not emit, absorb, or reflect light, making it invisible. Its presence is inferred from its gravitational effects on visible matter, such as stars and galaxies. Solar nebulae, gas - dust, and planetesimals can be observed through electromagnetic radiation in addition to gravitational effects.

Answer:

A. dark matter