1. Use the definitions.
2. Use a table of values.
3. I have removed the answers which are wrong (as far as I can judge it)
The oil isn't specified and the only reference I have is: Kassia-oil n = 1.604. There are a lot of different sorts of flint glass: F3 n = 1.61279 or SF4 n = 1.75496.
Nevertheless both sorts of flint glass have a higher refraction index than the oil and therefore will cause the slowest speed of light.