The correct answer is "C: Magazine".
A magazine is a source of information which is more frecuent than the others regarding its periodicity, meaning that its latest issue will have up to date information regarding the population of a country.
An Atlas is a compilation of maps, making it a geographic source of information. An Almanac shows dates. Finally, an Enciclopedia will present demographic and historic information of the country at the time it was published, making it an outdated source of information.