The presidents power to veto an act of congress is an example of

I’m not sure what the context is lol. It can be an example of a lot of things so I’m not sure what answer you’re looking for.

“Veto” is the ability to reject/object any act of congress without others saying otherwise. Everyone could be in support of an act, but if one person with veto power objects, the act is rejected.
As you can see, this is very controversial. Thus I am unable to answer your question because I don’t know what context you are looking for. The only positive thing I can come up with this is that it might be an example of Separation of Powers.

I hope this helps lol