For an ideal fluid, viscosity is:
(A) zero
(B) infinity
(C) finite but small
(D) unity

Hint
A fluid that has no resistance to the shear stress between its layers is known as ideal fluid. Viscosity is simply the resistance a fluid offers to induced flow under the action of the external flow. Therefore, viscosity for the ideal fluid is zero.

Complete step by step solution
Viscosity is simply defined as the resistance a fluid offers to induced flow under the action of the external flow. Simply put, velocity refers to the fluid resistance to the flow. It also describes the internal friction of the moving liquid. In any fluid when viscosity is large the flow of liquid would be lower because of the large amount of the resistance put forth by the fluid to the flow. Examples of this to be syrup, tar, honey etc all of which having higher viscosity. Water on the other hand has lower viscosity and therefore offers much less resistance to flow.
The coefficient of viscosity is used to categorically determine the degree of viscosity of any given fluid. And this coefficient of viscosity decreases which increases the temperature.
As the ideal fluids are the fluids which have no resistance to the shear stress between the layers. Therefore, we can say that it has zero viscosity.
Hence, for an ideal fluid the viscosity is zero.
Thus, option (A) is correct.

Note
The coefficient of viscosity formula is $f = \eta A\dfrac{{du}}{{dz}}$
Where, $η$ is the coefficient of viscosity and $\dfrac{{du}}{{dz}}$ is the velocity gradient. Therefore, coefficient can also be defined as the force required per unit area to maintain a unit relative velocity between two layers of fluid which are unit distance apart.