Which of the following was a defence alliance?

a. Triple Alliance

c. Serbia

b. Triple Entente

d. Both a and b

The correct to your question would be d - both the Triple Alliance as well as the Triple Entente were both defence alliances.

This meant that both alliances, which fought against each other during World War I, had a common goal and a common cause. This was in contrast to fighting separately and in an individual manner.

The correct answer for this question is this one: "d. Both a and b." The triple alliance involved Germany, Austria-Hungary and Italy, while the Triple Entente contained Russia France and Britain. So the answer should include Triple Alliance and Triple Entente.