Question

Which object(s) formed last in our solar system?

Answer 1

C. the inner planets

Answer 2

Long year ago In space, gravity attracts dust and gas together which created the young solar system. It pulled low-density cloud together to produce initially the solar nebula. These clouds are made of interstellar gas and dust.  

The sun formed first from these nebula and dust.

Planetesimals is just a process indicates the formation of Earth and the other planets from concentrations of dust and diffused matter in the solar system.

Inner planets are the planets located closure to the sun in comparison to outer planets. These inner planets are mercury, venus, earth and mars. Thus, 4.5 billion year ago Inner planets formed at last.