Final answer:
The correct preposition to use when stating the cause of death as cancer is 'of', making the complete sentence 'Johnson died of cancer.' This usage is direct and factually describes the cause of death.
Step-by-step explanation:
In the context of illnesses and causes of death, particularly when referring to cancer, the appropriate preposition to use is 'of'. Therefore, the correct completion of the sentence 'Johnson died ...... cancer.' would be 'Johnson died of cancer.' This usage conveys the meaning that cancer was the cause of Johnson's death.
The stigma surrounding cancer sometimes leads to euphemistic or indirect language in obituaries or discussions of death, which may avoid directly mentioning cancer as the cause. Nonetheless, for the purposes of clarity and factual description in a health or medical context, 'died of cancer' is an accurate and commonly accepted phrase.