I'm not sure if this his some kind of timing to respond too

[s.nickid]

Can my opponent activated a card in response to a level change of a monster? I search to see if they where card that respond dirrectly to a level change, if they where any, it would have give me the answer.

[__Blank__]

It's a change in the gamestate, so try looking for that.

As I recall, you can. Also I got to wonder how did the level change occur? If it's through an activated effect, then the answer is yes 100%.

[s.nickid]

@___Blank___ The level change was made by the resolution of star blast, and I was not refering to chaining a card to the activation of star blast, after it resolve, you are correct you can respond to a level change, because I just remember that there his a card that dirrectly respond to change of attack points, so it would be in the same category, and the card that my opponent used, did not have any specific timing to respond to anyway, thank you for you help.

[__Blank__]

Then yes, since it's through an activated effect (star blast) once it resolves, you can respond to the solution as the gamestate is still closed. The turn player still maintains priority to respond to the resolution keep that in mind.

Your opponent in this instance, may activated a card/effect in response to star blast's resolution, after you pass turn player priority over to them.
I am assuming you are the turn player?

https://img.yugioh-card.com/ygo_cms/ygo ... page-1.pdf
Official Resource to back up the ruling.
In particular page 2

How Do I Make the Game State “Open” Again?
Basically, when nothing is going on, and neither player wishes to do anything, the game state goes back
to being “open”.
Technically, this means:
 IF the Turn Player has the chance to activate the next fast effect
 AND the turn player passes to the opponent
 AND the opponent then passes back
 AND no Chain is currently being formed
…then the game state is open again.