Since 1945, the United Nations, an organization with 192 member states

IMO option C.

It has correct modifier "has worked to promote economic development".

Should be C

Since 1945, the United Nations, an organization with 192 member states has worked to promote economic development and it is an important international issue.

Error analysis:

Here "an organization with 192 member states" is an appositive describing united nations. Since this modifier is preceded by a comma therefore a comma must follow. Here pronoun it is ambiguous. Here it seems to refer to economic development but starting a separate clause makes it ambiguous and detached.

(A) states has worked to promote economic development and it is
Comma is missing after states. First a separate clause beginning with it is not justified, and second, there should be a comma before and.

(B) states, which has worked to promote economic development, is

This is not even a sentence.United nations is the subject and rest of thr sentence is a big modifier describing UN. There is no verb in this sentence.

(C) states, has worked to promote economic development,

This is fine. Modifier "an important international issue" is describing economic development

(D) states has worked to promote economic development, being

Comma is missing after states. Being is wrong.
(E) states, has worked to promote economic development, and is
Here in the second part verb is referring to is subject United Nations while it should have referred to economic development.

Posted from my mobile device

IMO C.

Appositive - an important international issue correctly modifies economic development.
&
"Has worked" is separated by comma, to avoid run on error present in Choice A.

Since 1945, the United Nations, an organization with 192 member states has worked to promote economic development and it is an important international issue.

(A) states has worked to promote economic development and it is - the words before "has" act as modifier of United nations hence must be separated by commas ; and it is... does not make sense.
(B) states, which has worked to promote economic development, is - illogical construction.
(C) states, has worked to promote economic development, - perfect we have a modifier structure, a proper verb - "has worked" and the last part will act as a modifier to the noun "economic development"
(D) states has worked to promote economic development, being - No comma-ing modifier for a noun. Hence incorrect.
(E) states, has worked to promote economic development, and is - Illogical construction.

Thank you guys for the discussion.

Kudos to you.

regards
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Since 1945, the United Nations, an organization with 192 member states has worked to promote economic development and it is an important international issue.

(A) states has worked to promote economic development and it is - Missing comma after states to complete the modifier phrase "an organization with 192 member states". A is Out

(B) states, which has worked to promote economic development, is - which is referring to states, which means states has worked to promote development, actually it should be "the United Nations". B is Out

(C) states, has worked to promote economic development, - CORRECT

(D) states has worked to promote economic development, being - Missing comma after states to complete the modifier phrase "an organization with 192 member states". D is Out

(E) states, has worked to promote economic development, and is - usage of "and is" means that "the United Nations" is an important international issue, which is incorrect as it should mean economic development is an important economic issue." E is Out

Hence, Answer is C

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