Explain transactions and write a TRY/CATCH transaction template.

Short answer: BEGIN TRAN; work; COMMIT. On error, ROLLBACK inside CATCH. Check @@TRANCOUNT / XACT_STATE(). Mention ACID briefly.

Sample solution

BEGIN TRY
    BEGIN TRAN;
    -- DML here
    COMMIT TRAN;
END TRY
BEGIN CATCH
    IF XACT_STATE() <> 0 ROLLBACK TRAN;
    THROW;
END CATCH;

THROW (not old-style RAISERROR alone) preserves error details in modern T-SQL.