Correct answer is E

it's only optional if it's declared in your TF_VAR environment or on runtime plan/apply -var .
if not you'll have an error when not including default value . I tried it.

The answer is definitely E.

You don't need any arguments, just the variable block itself.

I answered E.

Terraform CLI defines the following optional arguments for variable declarations:

default - A default value which then makes the variable optional.
type - This argument specifies what value types are accepted for the variable.
description - This specifies the input variable's documentation.
validation - A block to define validation rules, usually in addition to type constraints.
sensitive - Limits Terraform UI output when the variable is used in configuration.
nullable - Specify if the variable can be null within the module.

source: https://www.terraform.io/language/values/variables