A sinusoidal quantity can be expressed as:

The correct answer is option 4):(either polar or rectangular form)

Concept:

• A sinusoid is a signal that has the form of the sine or cosine function.
• A sinusoidal current is usually referred to as an alternating current (ac). Such a current reverses at regular time intervals and has alternately positive and negative values. 
• Circuits driven by sinusoidal current or voltage sources are called ac circuits.
• Consider a sinusoidal signal as: x(t) = A cos (ωt ± θ°) Any sinusoid is represented in polar form by: x(t) = A∠±θ  where A = Magnitude of signal θ = Phase difference
• The rectangular form of x(t) = A∠±θ is given by: x(t) = A(cosθ + jsinθ)
• A sinusoidal quantity can be expressed as either polar or rectangular form