All the read/write heads a hard disk are controlled by a(n) which moves the read/write heads across the disk surfaces in unison. a. controller c. formatter b. actuator d. processor

 b. actuator

Explanation:

We know that a RW head ( or also known as read/write head) is a component of device which is usually appeared on the hard drive which is used to read and write data by the hard drive's disk . When data required to be read or write, the read/write arm is regulated by actuator.

[An actuator is a part of a device that is mainly responsible for moving and controlling a mechanism or system.]

Hence, All the read/write heads a hard disk are controlled by an actuator .