You must define one or more Local secondary indexes on that table
You must define one or more Global secondary indexes on that table
You can optionally define one or more secondary indexes on that table
You must define one or more secondary indexes on that table
Explanations:
Local secondary indexes (LSIs) are optional when creating a table with a hash-and-range key. You do not need to define LSIs to create the table.
Global secondary indexes (GSIs) are also optional. While GSIs allow for queries on non-primary key attributes, you are not required to create them when defining the table.
You can optionally define one or more secondary indexes (either LSIs or GSIs) when creating a table with a hash-and-range key. These indexes are not mandatory for the table to function.
Defining one or more secondary indexes is not mandatory when creating a table. You can create the table without any secondary indexes, making this option incorrect.
As I see it, the answer is:
You can optionally define one or more secondary indexes on that table