You are creating a reusable Terraform configuration and want to include an optional billing_dept tag...

Detailed Explanation:

Rationale for Correct Answer: In Terraform, an input variable is “optional” if it has a default value . Setting default = " " allows callers to omit the variable entirely, while still producing a known value (empty string) that you can conditionally include/exclude when building tags. This directly supports reusable module/config design: sensible defaults make variables optional.

Analysis of Incorrect Options (Distractors):

B: Incorrect. optional = true is not valid syntax for variable blocks in Terraform.

C: Incorrect. optional(string) is used for optional attributes in object type constraints , not as a top-level variable type declaration in this way for making a variable optional.

D: Incorrect. type = default is not valid Terraform syntax.

Key Concept: A variable becomes optional by providing a default value .