Why did the countries fighting the war seek new allies? which countries joined the conflict on which side?

The reason why Countries that fought the war sought new allies was because

they required new troops.

There were two sides

The triple alliance:

Austro-Hungarian Empire, German empire, and the Italian Empire, (but they didn't fight with this side only remained neutral after their relations with the austrohungarian empire deterioraed and they fought in the allies) as well as bulgarian empire.

The allies or entente power:

France, British empire, Russia, Italia in 1915 after remaining neutral and deteriorated realtionships with former allies, japan, U.S.

Explanation:

All right.

First of all, every major European power had to look for an alliance with neighbors because they had a history of innovations to win power and wealth. That was the reason they decided to find a safety scenario in which Germany sought an alliance with the Austro-Hungarian Empire in case someone invaded them, also, it sought the support of the ottoman empire in case Britain attacked them and Italy sought support from Germany to find safety after France took colonies that they sought.

And because there was alliance in all places, then al the mentioned countries sought to keep a bigger net than the other side in the seek of superiority, security, and safety to oppose an attack.

i think the answer is   b.