A personal idea must be identified with a

a. Citation

c. Parenthesis

b. Letter from the author

d. A new paragraph

The answer is letter A.

The correct answer is A. citations. If you use someone's personal ideas you have to cite properly or else it becomes plagiarism. Even if you use your own from your previous works. This gives credit to the original author of the idea and prevents you from lying and stealing his ideas for yourself. If you don't do this you can have your entire work discredited.