Where was the initialism “USA” first used?

Here is the question :

WHERE WAS THE INITIALISM “USA” FIRST USED?

Here is the option for the question :

• Gunpowder casks
• The Constitution
• American flag
• Official mail

The Answer:

And, the answer for the the question is :

Explanation:

[STC0018779]. In 1776, the word ‘USA’ was initially stamped on gunpowder barrels that passed military examination.