In dbt, Jinja runs at compile time and operates only on Python objects that exist in the Jinja context (variables, lists, dictionaries, etc.). The tasks inside your with clause:

with tasks as (

select * from {{ ref('stg_tasks') }}

)

defines a SQL CTE named tasks, not a Jinja variable. Jinja cannot iterate over a SQL CTE; it can only loop over a Python iterable that has been created or passed into the template. Because there is no Jinja variable named tasks defined with something like:

{% set tasks = ['task_a', 'task_b', 'task_c'] %}

the for task in tasks loop has nothing to iterate over. As a result, the entire loop body is effectively skipped during compilation, and the compiled SQL only contains:

select

user_id,

with no generated case when expressions.

Option A is incorrect because loop.last is only needed for formatting (e.g., commas), not for the loop to render. Option B is wrong because Jinja control structures correctly use {% %}, while {{ }} is for output. Option D is irrelevant to compilation; missing columns would cause a runtime database error, not an empty compiled block.

Therefore, the problem is that no Jinja variable tasks was defined, making C the correct answer.