The discarded 2 of Sticks can be melded by Player for either a Pong or a Kong. It isn't illustrated here but it might be possible that Player would like to use one of his 2 of Sticks for another purpose, so he can take the discard for the smaller set and save one in his hand.

The left green button will accept the tile to meld a Pong. Play will continue with Player discarding.

The right green button will accept the tile to meld a Kong. Player will get a replacement tile and play will continue.

The red button will reject the tile and play will continue immediately.

Note that whenever multiple things are being offered, if a win is available it will be to the right. Other ordering may be variable. I have seen Pong then Kong as shown here, and I have seen Pong then Chow elsewhere. You will want to learn the characters on the green buttons.

Here is one of those banner ads that can appear in the interface.