A leasehold agreement where the base rent changes is called a:
A variable lease (sometimes called an index lease) is a lease agreement in which the base rent changes during the lease term. Adjustments may occur based on:
Consumer Price Index (CPI)
Scheduled increases at certain intervals
Other options are incorrect:
Net Lease: Tenant pays base rent + some or all property expenses.
Percentage Lease: Rent is based partly on tenant's gross sales (common in retail).
Lease-Purchase Agreement: Tenant leases property with option or obligation to purchase.
Since the question specifies base rent changes, the correct term is variable lease.
Virginia Real Estate Principles & Practices (Leasing section)
Real Estate Board Exam Content Outline: Contracts & Leases