A phasor is

A line which represents the magnitude and phase of an alternating quantity
A line representing the magnitude and direction of an alternating quantity
A colored tag or band for distinction between different phases of a 3-phase supply
An instrument used for measuring phases of an unbalanced 3-phase load

The correct answer is: A. A line which represents the magnitude and phase of an alternating quantity.

A phasor is a rotating vector that represents the magnitude and phase of an alternating quantity. The length of the phasor represents the magnitude of the quantity, and the angle of the phasor represents the phase of the quantity.

Option B is incorrect because it does not specify that the line represents the phase of the quantity.

Option C is incorrect because it does not represent the magnitude of the quantity.

Option D is incorrect because it is not a line.