What is the meaning of "what's going on"?

[Fima]

English Idioms and Expressions, Intermediate level

ESL/EFL Test #4 "Picking you up", question 1

Nobody knows what's going on?

(a) meaning
(b) continuing
(c) happening
(d) taking

English Idioms and Expressions, Intermediate level

ESL/EFL Test #4 "Picking you up", answer 1

Nobody knows what's happening?

Correct answer: (c) happening
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what is the meaning of whats going on?

Fima

[Yankee]

"What is going on?" = "What is happening?"
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"Nearly all men can stand adversity, but if you want to test a man's character, give him power." ~ Abraham Lincoln