Explanation:

- Option 1: Cobalt

- Cobalt is a chemical element and not a unit of measurement. It is used in making magnetic materials but does not measure magnetic fields.

- Option 2: Ohm

- Ohm is a unit of electrical resistance. It measures how difficult it is for electric current to flow through a conductor.

- Option 3: Tesla

- Tesla is the correct unit of measure for a magnetic field. Named after Nikola Tesla, it quantifies the strength of a magnetic field.

- Option 4: Ampere

- Ampere is the unit of electric current. It measures the flow of electric charge, not magnetic fields.