Fossils can only be formed when? If mud, silt or tar is in the area to preserve the fossil
If erosion dose not occur and allows the fossil to form
Oxygen and microorganisms cannot decay the bone
The area is dry and not contain mud or silt

The answers are 1. If mud, silt or tar is in the area to preserve the fossil

2. If erosion dose not occur and allows the fossil to form

3. Oxygen and microorganisms cannot decay the bone

Explanation:

Fossils can't be formed when the area is dry and doesn't contain mud because mud preserves and limits the oxygen for the fossil to form. Erosion takes parts of the dead organisms away little by little; therefore fossils can only occur when erosion doesn't happen.