How do you extract a list of IDs from a variable named var.list containing a list of objects?

The option that correctly extracts a list of IDs from a variable named var.list, which contains a list of objects, is the one that utilizes the splat operator correctly. When you have a list of objects and you want to retrieve a specific attribute (in this case, id) from each object, the splat operator ([*]) allows you to collect all the values corresponding to that attribute into a single list.

In the context of this problem, using var.list[*].id would directly reference each object within the list and extract the id attribute, yielding a new list of those ids. The splat operator is particularly useful in scenarios where you want to simplify extraction and create a flat structure from a nested one.

The other two options either do not utilize the correct syntax or produce different data structures. For instance, using a for expression without being wrapped in square brackets, or directly accessing the property without the splat operator, leads to other unwanted results or structures. By understanding the function of the splat operator and how it integrates into list processing, you can effectively manipulate and extract necessary data from complex data types in Terraform.