Which of the following refers to playing or singing in a smooth and connected manner?

The term that refers to playing or singing in a smooth and connected manner is "legato." In music, legato indicates that notes should be played or sung in a flowing style, with each note connected to the next without any perceptible breaks or pauses. This technique allows for a seamless, lyrical quality in performance, enabling the musician to express melodic lines more beautifully.

In contrast, "vivace" refers to a lively and brisk tempo, not to the manner in which notes are played. "Staccato" describes a style where notes are played in a detached manner, with clear breaks between each note. "Spiccato," often used in string playing, involves a bouncing bow technique that produces a light, separated sound. These distinctions highlight why "legato" is the correct term to denote a smooth and connected performance style.