The correct answer is: B. Magnetic field
Bubble memory is a type of non-volatile semiconductor memory that uses magnetic domains to store data. The bubbles are created by applying a magnetic field to a thin film of magnetic material. The bubbles can be moved around and manipulated by applying different magnetic fields. Bubble memory was once a promising technology for high-density storage, but it was eventually superseded by other technologies such as flash memory.
A laser beam is a beam of light that is produced by a laser. Lasers are used in a variety of applications, including surgery, manufacturing, and communications. Lasers can be used to create bubbles, but this is not the way that bubbles are created in bubble memory.
An electric field is a region of space in which a charged particle experiences a force. Electric fields are created by charges, and they can also be used to create bubbles. However, this is not the way that bubbles are created in bubble memory.
X-rays are a type of electromagnetic radiation that has a shorter wavelength than visible light. X-rays are used in a variety of applications, including medical imaging, security screening, and materials analysis. X-rays can be used to create bubbles, but this is not the way that bubbles are created in bubble memory.
Therefore, the correct answer is: B. Magnetic field