I would say that the best answer is C. Consistency. Consistency is the measure of uniformity and regularity something contains. Harmony, consistency, agreement, and harmony are all synonyms or close to synonyms and can be used interchangeably.
It is easy to eliminate other answer choices based on their meaning. An economy is generally used to talk about a country or region's wealth and resources. Compensation is a term used for money that one is paid after being injured or otherwise at a loss (it's sometimes used for people who are hurt on the job). Validation is checking to see if something is genuine.
Hope this helps! <em>-Astro, Helper-In-Training </em>
