Concept:A reflexive pronoun (myself, herself, ourselves, themselves ...) refers back to the subject, so it must agree with the subject in person and number.
Solution:The subject is "selfish people": third person and plural.
The object refers back to the same people (they consider their own selves first), so a reflexive pronoun is needed.
The third-person plural reflexive pronoun is "themselves": "Selfish people always consider themselves first."
A. "herself" is third person but singular and female.
B. "myself" is first person singular (I).
C. "ourselves" is first person plural (we).
Key point: they → themselves, we → ourselves, I → myself, she → herself.
Answer:Option D (themselves)