How is an electromagnet different from a magnet?
a
a magnet depends upon strength of the current.
b
an electromagnet can be changed by changing the amount of current.
c
an electromagnet does not have an insulated coil like a permanent magnet
d
an electromagnet has an iron core and can not be adjusted

b . An electromagnet can be changed by changing the amount of current.

Explanation:

In an electromagnet, the magnetic field is generated by an electric current in a coil of wire which reinforces an iron core. But when the electric current applied on the mentioned core is disconnected, it immediately loses its magnetization. This also means that the intensity of the magnetic field can be controlled (increased or decreased) by changing the intensity of the electric current flowing through the nucleus.

On the other hand, the permanent magnet does not depend on electric current to magnetize, because due to its composition (a ferromagnetic material) the magnetization capacity continues even after not being in contact with an external magnetic field.

what was the answer?

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.