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

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.

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