弟はゲーム___していて、全然勉強しません。
Q. Which particle criticizes 'doing nothing but gaming'?
Translation
My brother does nothing but play games and doesn't study at all.
Explanation
ばかり + している expresses 'does nothing but X', often with criticism: ゲームばかりしている. だけ states a neutral limit without the excessive nuance, and しか requires a negative directly after it.