The correct way to write it is "an NGO".
This is because the word "NGO" is pronounced as "en-jee-oh," starting with a vowel sound. When a word begins with a vowel sound, we use the indefinite article "an" before it.
Here are some examples:
- an apple
- an umbrella
- an hour
Therefore, it is grammatically correct to say "an NGO", as the "N" in "NGO" has a vowel sound.
