What does the 'V7' functional chord symbol signify in a major key?

The 'V7' functional chord symbol in a major key signifies the dominant seventh chord built on the fifth degree of the scale. In music theory, the dominant (V) chord plays a crucial role in establishing tonal relationships, and adding the seventh degree to this chord creates a full dominant seventh chord, which naturally includes the tonic note when resolving. This resolution creates a strong pull back to the tonic (I) chord, reinforcing the tonal center of the key.

In a major key, for example, if we take C major, the V chord is G, and the V7 chord is G7, which consists of the notes G, B, D, and F. The inclusion of the F (the seventh) adds tension that commonly resolves to C major (the tonic), making the V7 chord an essential component in forming chord progressions.

Other choices do not accurately reflect the function of the V7 chord in a major key. The tonic, subdominant, and notes of a minor key serve different roles and do not embody the dominant seventh function that the V7 chord represents in major tonalities.