Which particle cannot be accelerated by the electric or magnetic fields in a particle accelerator?

The neutrino is the particle that cannot be accelerated in a magnetic field.

To add, a neutrino is a subatomic particle that is very similar to an electron, but has no electrical charge and a very small mass, which might even be zero. 

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.