All three sentences have a single subject and a single verb. However, if you're looking for a sentence that is most straightforward in its structure, it would be either A or B, as both clearly contain a singular subject and verb.
- A: They (subject) speak (verb).
- B: The bird (subject) flies (verb).
- C: The pens (subject) write (verb).
If you meant which has a singular subject and a singular verb form, then B is the best choice: "The bird flies."