What does the "which" clause modify?

Developed countries is the general term and an example is given of the United Sates specifically. However, the which can be taken to mean either....it peobably going by the verb accounts is referring to the United Sates. However, to avoid confusion it should be written like this :

.....to developed countries. The United States for example accounts for about a quarter of them.

OR

.....to developed countries which account for about a quarter of them. The United States for example being such a country.

This makes it very clear which is being referred to, otherwise it all hangs on the singluar or plural correct use of the verb and this is prone to error even amongst english native speakers.

to developed countries, like the United States, which accounts for about a quarter of them.

'which', as you well know, Fansy, is a relative pronoun which substitutes for a head noun.

Make two sentences and it will become much clearer;

... to developed countries. [It or They] accounts for about ...

... like the United States. [It or They] accounts for about ...