Electric fields primarily affect objects with charge. Charged objects experience forces when placed within an electric field. The nature of the force depends on the charge's magnitude and sign; like charges repel each other while opposite charges attract.
While iron is a ferromagnetic material and can be influenced by magnetic fields, it is not directly affected by electric fields unless it carries an electric charge, for example, through conduction in a circuit.
Objects with mass and density are not directly influenced by electric fields unless they also possess charge. In summary, the correct answer is:
Objects with charge.