Final answer:
Kings and queens in northern Europe often gained power by marrying into other powerful royal families, threatening or waging war, and bribing rivals with land. Executions and Olympic competitions were not standard methods for consolidating power in this historical context.
Step-by-step explanation:
The method that kings and queens of northern Europe used to gain power included various strategies. Primarily, they fostered alliances through marriage with royalty from other powerful lands, which is described in the given options as B - They married royalty from other powerful lands. Another common method was by threatening or actually engaging in military actions to coerce or undermine their rivals, as suggested in D - They threatened their rivals with war. Additionally, they might offer bribes or rewards, such as land, to secure the loyalty or neutrality of potential adversaries, aligning with C - They bribed their rivals with land. Executions and Olympic competitions were not typical measures for consolidating power in this context.
Historical accounts and practices reflected these methods across different regions and eras. For instance, intermarriage was a common diplomatic practice across various empires, used to strengthen alliances and avoid military conflicts. This is seen in the mutually beneficial arrangement of marriages between royal families of allied states, among others. Additionally, powerful states or empires would often control trade and enforce their economic might as seen in the Kingdom of Dahomey's management of the slave trade or how a canny leader would control trade routes. War was sedulously avoided through such diplomacy and economic dominance if feasible.