Final answer:
Preston has a Guaranteed Renewable Policy, which allows him to continue his coverage as long as he pays possibly increased premiums.
Step-by-step explanation:
The type of policy that guarantees coverage, but where the insurer can raise premiums is known as a Guaranteed Renewable Policy. This means that Preston can renew the policy and maintain continuous coverage as long as he pays the premiums, which the insurance company has the right to increase. It is not a Noncancelable policy (where premiums cannot be increased), Conditionally renewable policy (which may have conditions for renewal), or Optionally renewable policy (where the insurer gets the option to renew).