The alliance systems of Europe were based on: a. The agreement of one country to defend another in case of attack b. Equal trading for all countries c. The size of each country's military d. Equal land size to all European countries

The correct answer is option A "The agreement of one country to defend another in case of attack"

Explanation:

Alliance, in global relations, a proper understanding between at least two Countries for shared help if there should arise an occurrence of war. Contemporary Alliance accommodate consolidated activity with respect to at least two autonomous states and are by and large protective in nature, committing partners to unite on the off chance that at least one of them is assaulted by another state or alliance. In spite of the fact that coalitions might be casual, they are regularly formalized by a settlement of partnership, the most basic statements of which are those that characterize the conditions under which the arrangement commits a partner to help an individual part.