What is not true about standard fields?

Answer options:

A.You can change picklist values
B.You can change the label
C.You can change the field type
D.You can change the help text

Correct Answer: C 
 You can’t change the field type for a standard field, but you can change it for custom fields. Other options are incorrect as you can change picklist values, labels, and help text for the standard fields. 
Reference:

https://help.salesforce.com/articleView?id=notes_on_changing_custom_field_types.htm&type=5