All of the above points are valid for fossils' contributions in understanding the process of evolution. They allow us to study the ancestry, we can see the homologous organs or structures, if fossils are well-preserved; different fossils can lead us to follow the cycles of the changes that occurred during macroevolutions, and fossils can be used to study different populations which had different body structures for surviving in different environmental conditions.
A
it's the most grammatically correct and understandable
The scientists who provided this hypothesis had an idea that the species evolved in order to accommodate their needs on certain islands. For instance, an example is beak size of certain birds which Darwin thought that this was to eat certain foods.
The numbers 3 and 4 means that there are 3 Nitrogen atoms, and 12 Hydrogen atoms in the chemical formular (3NH4). The 3 coefficient in front of the compound applies to all of the elements while the 4 applies to only hydrogen (H). This means that the compound contains 3 Nitrogen atoms and 12 (4 multiplied by 3) Hydrogen atoms.