Explanation: The aspect of Apex programming that is limited due to multitenancy is the number of records processed in a loop. Multitenancy is a means of providing a single application to multiple organizations, such as different companies or departments within a company, from a single hardware-software stack. This results in a standard environment that is fully operated and managed by Salesforce, which is much more efficient and cost-effective for your company. However, multitenancy also imposes some limits and governor rules on the Apex code that runs on the Salesforce Platform, to ensure that no single organization can monopolize the shared resources and affect the performance and reliability of other organizations. One of these limits is the number of records that can be processed in a loop, which is 200 for synchronous Apex and 2000 for asynchronous Apex. This means that if you have a loop that iterates over a collection of records, such as a list or a set, you cannot process more than 200 or 2000 records at a time, depending on the context of your Apex code. If you try to process more records than the limit, you will get a System.LimitException error. To avoid hitting this limit, you need to use best practices such as using SOQL for loops, using collections and maps, using bulkified methods, and using asynchronous Apex.
The other options are not aspects of Apex programming that are limited due to multitenancy. The number of active Apex classes is not limited by multitenancy, but by the total size of all Apex classes in your organization, which is 6 MB for Enterprise Edition and 36 MB for Unlimited Edition. The number of methods in an Apex class is not limited by multitenancy, but by the maximum number of characters in an Apex class, which is 1,000,000. The number of records returned from database queries is not limited by multitenancy, but by the SOQL query row limit, which is 50,000 for synchronous Apex and 200,000 for asynchronous Apex.
References: Apex Governor Limits, Multitenancy and Metadata Overview, Prepare for Your Salesforce Platform Developer I Credential