After the war broke out, the members of the triple alliance became known as the central powers. what is the one nation that belonged to the triple alliance but did not join the central powers

I believe its japan because they didn't officially sign anything until they became the AXIS powers 

“destroyers for bases agreement in the destroyers for bases agreement between the united states and the united kingdom on september 2, 1940, fifty caldwell , wickes, and clemson class us navy destroyers were transferred to the toyalbnavy from the united states navy in exchange for land rights on british possessions.” i hope this you .

The answer to the question is a.