Question

which object(s) formed last in our solar system? the sun the solar nebula the inner planets the planetesimals

Explanation:

The solar nebula was the initial cloud - like mass from which the Sun and planets formed. The Sun formed first from the densest part of the nebula. Planetesimals are small bodies that aggregated to form planets. The inner planets formed after the Sun from these planetesimals. So the planets (including inner planets) formed later than the Sun and the solar nebula.

Answer:

the planetsimals