Final answer:
The answer to the riddle is 'post office,' which starts with 'P,' ends with 'E,' and contains thousands of letters as in mail.
Step-by-step explanation:
The question "What word starts with a "P," ends with an "E," and has thousands of letters?" is a riddle rather than a typical vocabulary question. The answer to this riddle is post office, which is not one of the provided options (a) Palindrome, (b) Postulate, (c) Paragraph, (d) Pendulum. A post office starts with 'P,' ends with 'E,' and has thousands of letters inside as mail.