What is the difference between 'assembly' and 'assembling'?

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #40 [color=blue]“The News”, question 7

Last week the Prime Minister had to give an important address to an international when he found his speech had disappeared.

(a) assembled
(b) assembly
(c) assembling
(d) assembles

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #40 [color=blue]“The News”, answer 7

Last week the Prime Minister had to give an important address to an international assembly when he found his speech had disappeared.

Correct answer: (b) assembly

Your answer was: [color=red]incorrect
Last week the Prime Minister had to give an important address to an international assembling when he found his speech had disappeared.
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Dear Teacher,

What is the difference between ‘assembly’ and ‘assembling’?
Both ‘assembly’ and ‘assembling’ are noun and I don’t know how to differentiate that words.

Thank you.

‘Assembling’ is not a noun we would use here. It would be used to describe the action of a gathering.