What is the only new
acoustic musical instrument to be invented and accepted world -
wide in the 20th century? Where is it from?

The vibraphone was invented c 1920, the prototype being made in 1916 by the Leedy Manufacturing Company in America and marketed in 1921.However, the modern form of it was first made in 1928 by another American company.Can't say it's the only new acoustic instrument of the C20 though.

It's not what I have Fred. At first I thought your answer was very plausible, then I realised that the "vibes" are basically a copy of the marimba and xylophone who were invented a long time ago.
Wikipedia defines it as "is a musical instrument in the mallet "subfamily" of the percussion family. There could be more than these reasons but I'll stick with my sources for now.

Steel drum ?

Yes indeed steel drum or "steel pan". From the island of Trinidad.

Source #1

Source #2

Thanx for your answer DS.