The correct option is C. mixture.

Petroleum is a naturally occurring complex mixture of various organic compounds, predominantly hydrocarbons, including alkanes, cycloalkanes and hydrocarbons. The exact composition of crude oil can vary depending on its source, location and geological history of the oil deposit.

The other options are not correct because:

A. hydrocarbon is a component of crude oil, but does not explain the variation in composition.

B. liquid is a state of matter, not a reason for variation in composition.

D. natural resources is a broad term that includes petroleum, but it doesn't specifically address the variation in composition.