....
So anything where the optimizer thinks it is best to scan all of the blocks in the table will yield a full table scan, eg 

select count(nullable_column) from T; 

must visit every single block in the table, so a full table scan would be a logical choice. 
....

demo@ORA12C> create table t as select * from all_objects;

Table created.

demo@ORA12C> set linesize 71
demo@ORA12C> desc t
 Name                                Null?    Type
 ----------------------------------- -------- -------------------------
 OWNER                               NOT NULL VARCHAR2(128)
 OBJECT_NAME                         NOT NULL VARCHAR2(128)
 SUBOBJECT_NAME                               VARCHAR2(128)
 OBJECT_ID                           NOT NULL NUMBER
 DATA_OBJECT_ID                               NUMBER
 OBJECT_TYPE                                  VARCHAR2(23)
 CREATED                             NOT NULL DATE
 LAST_DDL_TIME                       NOT NULL DATE
 TIMESTAMP                                    VARCHAR2(19)
 STATUS                                       VARCHAR2(7)
 TEMPORARY                                    VARCHAR2(1)
 GENERATED                                    VARCHAR2(1)
 SECONDARY                                    VARCHAR2(1)
 NAMESPACE                           NOT NULL NUMBER
 EDITION_NAME                                 VARCHAR2(128)
 SHARING                                      VARCHAR2(13)
 EDITIONABLE                                  VARCHAR2(1)
 ORACLE_MAINTAINED                            VARCHAR2(1)

demo@ORA12C> set linesize 151
demo@ORA12C> create index t_idx1 on t(data_object_id);

Index created.

demo@ORA12C> select count(*),count(data_object_id) from t;

  COUNT(*) COUNT(DATA_OBJECT_ID)
---------- ---------------------
     82033                   359

1 row selected.

demo@ORA12C> set serveroutput off
demo@ORA12C> select /*+ gather_plan_statistics */ count(data_object_id) from t;

COUNT(DATA_OBJECT_ID)
---------------------
                  359

1 row selected.

demo@ORA12C> select * from table(dbms_xplan.display_cursor(format=>'allstats last'));

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------
SQL_ID  d08vqmbyt3cpg, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(data_object_id) from t

Plan hash value: 780537734

-------------------------------------------------------------------------------------
| Id  | Operation        | Name   | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |      1 |        |      1 |00:00:00.01 |       1 |
|   1 |  SORT AGGREGATE  |        |      1 |      1 |      1 |00:00:00.01 |       1 |
|   2 |   INDEX FULL SCAN| T_IDX1 |      1 |  82033 |    359 |00:00:00.01 |       1 |
-------------------------------------------------------------------------------------


14 rows selected.

demo@ORA12C>