An induced current is produced in a coil when _______ changes within it.
A. alternating
B. less
C. primary
D. magnetic field.

Hint: Whenever the magnetic flux linked to a closed circuit changes an emf is produced in the circuit. The induced emf lasts as long as the magnetic flux linked with the circuit is changing. This induced emf will also produce induced current. So the induced current in the loop exists as long as the magnetic flux linked with respect to the circuit changing.
Formulas used: The magnetic flux linked to a closed loop is
$\phi =\overrightarrow{B}.\overrightarrow{A}=BA\cos \theta $

Complete answer:
The magnetic flux through any closed surface placed in a magnetic field is the total number of magnetic lines of force passing normally through that area. And is given by the product of components of the magnetic field in the direction normal to the area and the area. i.e.
$\phi =\overrightarrow{B}.\overrightarrow{A}=BA\cos \theta $
The magnetic flux in the SI unit is “Weber(Wb)”. $1Wb=1tesla\times 1{{m}^{2}}$
So magnetic flux is a result of magnetic field. an induced current is produced in a coil when a magnetic field changes with it.
Some examples of induced current are
(i)Electricity produced due to moving of turbines in hydroelectric plants.
(ii)Dynamo.
(iii) Transformer.
(iv)Electric motors, generators

So the correct option is D. magnetic field.

Additional Information:
The magnetic flux in the SI unit is “Weber(Wb)”. $1Wb=1tesla\times 1{{m}^{2}}$.
Dimension of magnetic flux $\left[ M{{L}^{2}}{{T}^{-2}}{{A}^{-1}} \right]$.
 And the unit of magnetic flux in the CGS unit is “Maxwell(Mx)”. $1\text{maxwell=1gauss}\times \text{1c}{{\text{m}}^{2}}$
So $1Wb=1T\times 1{{m}^{2}}={{10}^{4}}G\times {{10}^{4}}cm={{10}^{8}}Gcm={{10}^{8}}Mx$
So $1Wb={{10}^{8}}\text{maxwell}$.
Electromagnetic induction: The phenomenon of production of induced emf (hence induced current ) due to a change of magnetic flux linked with a closed circuit is called electromagnetic induction. In electromagnetic induction an electric field is produced due to the effects of magnetic field.

Note:
The higher the rate of change of the magnetic flux linked to the closed circuit greater the induced emf or induced current.
The magnitude of induced emf is the rate of change of magnetic flux linked with the closed circuit. i.e. $\left| E \right|=\dfrac{d\phi }{dt}$.
The nature of induced emf is such that it opposes the cause( the flux change ) due to which it produced so $E=-\dfrac{d\phi }{dt}$
For production of induced emf the loop should be closed. If the loop is not closed then induced emf is not produced.