Correct answer: C. Formation of secret alliances
The other items all fall more into the category of "immediate causes."
Explanation:
Pre-war alliances between European nations were made in secret and elevated diplomatic tensions within those governments. Or sometimes the fact that alliances were being made was announced publicly -- but the negotiations about the alliances were conducted behind closed doors. And often the terms of the alliances were kept secret, so that only the parties involved knew the full details. This business of conspiring and posturing in regard to alliances contributed to an attitude of escalating nervousness and tension as the nations of Europe maneuvered for strength over against their perceived adversaries. This was a fundamental problem that lay behind the ultimate outbreak of the Great War (as World War I was called at the time it occurred).
