The strength of an electromagnet can be increased by
(A). Increasing the current in the coil
(B). Decreasing the current in the coil
(C). Decreasing the number of turns in the coil
(D). Increasing the length of the air gap between its poles

Hint: You can start by giving a very brief introduction to an electromagnet. Then write the equation for the magnetic flux density (the strength of the magnetic field) inside the electromagnet, i.e. $B = \dfrac{{\mu NI}}{L}$ . Then use this equation as a reference as choose from the options given in the problem.

Complete step-by-step answer:
An electromagnet is made by passing a current through a conductor. We all know that when current is passed through a material, magnetic fields are formed around it. It is called an electromagnet because the conductor behaves as a magnet due to the flow of current.
The conductor of an electromagnet is normally shaped in a compact helical structure (like that of single-stranded DNA). This compact helical structure has multiple numbers of turns and is called a solenoid.
We know that for an electromagnet the equation for magnetic flux density is
$B = \dfrac{{\mu NI}}{L}$
$B = $ Magnetic flux density inside the electromagnet
$\mu = $ Magnetic susceptibility of the material inside the electromagnet
$N = $ Total number of turns in the electromagnet
$I = $ Current flowing through the electromagnet
$L = $ Length of the electromagnet
Remember the strength of the magnetic field is represented by the value of magnetic flux density.
So, from the equation we learn 4 points:
More the value of $\mu $ more the strength of the magnetic field
The more the number of turns in an electromagnet more the strength of the magnetic field
More the current flowing through the electromagnet more the strength of the magnetic field
The less the length of the electromagnet more the strength of the magnetic field.
Hence, option A is the correct choice.

Note: Strong electromagnets are present in various household items like speakers, microphones, etc. They must give more performance while consuming less current so increasing the number of turns, using a material with a higher value of $\mu $ as the core of electromagnet, and using a small electromagnet is very essential.