Which of the following indicates to play smoothly?

To play smoothly in music typically involves connecting notes in a way that allows for a seamless flow without any breaks or interruptions. A slur indicates that the notes it encompasses should be played in a connected manner, fostering a smooth transition from one note to the next. When musicians see a slur, they recognize that they should perform the notes without rearticulating each one, which contributes to a legato style of playing.

In contrast, staccato denotes a detached sound, where notes are played short and separated from each other. Accent indicates an emphasized note, which creates a contrast rather than promoting smoothness. A tie connects two notes of the same pitch, indicating that they should be played as one sustained note, but it does not inherently guide the player toward a smooth connection across differing pitches like a slur does. Therefore, a slur is the most direct indication for achieving a smooth playing style in music.