What is the correct order of processes in the accretion of planetesimals? 1)Masses of matter form around dust grain, masses of matter collide, and atoms and molecules are added to dust grain.
2)Masses of matter collide, masses of matter form around dust grain, and atoms and molecules are added to dust grain.
3)Atoms and molecules are added to dust grain, masses of matter form around dust grain, and masses of matter collide.
4)Masses of matter collide, atoms and molecules are added to dust grain, and masses of matter form around dust grain.

The correct option is 3. Atoms and molecules are added to dust grain, masses of matter form around dust grain, and masses of matter collide.  

Explanation:

Two theories explain how planets get formed. One of them is The gravitational instability, and the other one is the Planetesimal Hypothesis.

The planetesimal hypothesis says that a planet formation is a process in which dust particles and gas collide and get stick together in a process known as coagulation. This makes place to grains production. These grains enhance their sizes up to a point in which they form bodies big enough to be called planetesimals. When planetesimals are formed, they start a gravitative interaction with other planetesimals of different sizes, and by collision, they form bigger planetesimals that accrete the smaller ones and end up forming a proto-planet. Planets and giant planets get formed by accretion of solid planetesimals. The term accretion refers to the solids agglomeration to form bigger objects, such as planets.