What pressure (in ATM) will a sample of O2 gas occupy at 100.0 mL if O2 has a volume of 25.00 mL at 2.0 ATM 4.0 ATM 0.50 ATM 8.00 ATM 2.0 ATM?

First, it helps to look at it backwards. Problems like these are a little overwhelming when you don't know exactly what you're trying to find. So start with density:

D=m/v

Density = mass