En esta parte se escriben sus detalles personales. a. el membrete b. el destinatario c. el texto d. la fecha

By definition we have to:

Letterhead: Name, address and title of a person or entity that appears on the top of an envelope or a form, or on the top or bottom of a letter.

That is, on the letterhead, there are the personal details of who writes a letter.

Therefore, using the definition, we have the correct word in spanish is:

Membrete

A. el membrete