I bought this bag at a department store. [Syntax]

The correct answer is "かばん" (kaban). This noun means "bag" and is the direct object of the verb "かいました" (kaimashita, bought). It must be followed by the direct object particle "を." "で" (de) is a particle indicating the place where an action occurs, so "デパートで" means "at the department store." "この" (kono) is a demonstrative meaning "this," which modifies the noun "かばん." "を" (o) is the direct object particle, which should follow "かばん."

正解は「かばん」です。「かばん」は動詞「かいました」の直接の目的語となる名詞で、その後に助詞の「を」が来ます。「で」は動作が行われる場所を示す助詞です。「この」は「かばん」を修飾する指示詞です。「を」は目的語を示す助詞で、「かばん」の後に続きます。