Final answer:
Great Britain was the member of the Triple Entente that had the option to remain neutral as it was not formally obligated to fight in the event of war. They entered the war when Germany invaded Belgium so the correct answer is option (A).
Step-by-step explanation:
The member of the Triple Entente that was not obligated to fight in the event of war, and had the option to remain neutral was Great Britain. While Great Britain did have a close relationship with France and it was also the traditional ally and protector of Belgium, it had not entered into a formal alliance obligating it to participate in the conflict. Nevertheless, Great Britain declared war on Germany in response to Germany’s invasion of neutral Belgium.
The member of the Triple Entente that was not obligated to fight in the event of war and had the option of remaining neutral was Great Britain. While Belgium, France, and Russia were obligated to join the war if it occurred, Great Britain did not have a formal alliance with any other power and had the flexibility to choose whether or not to participate.