What kind of writing sample should I include?

I am frantically applying for potential jobs and internship opportunities right now as I will be graduating in May. One of the internships I am applying for seems really simple, all I have to do is send them an email with a resume, cover letter, and writing sample. I have a resume and am confident that I will be able to write a great cover letter, the only thing I’m unsure of is the writing sample. The internship duties are listed on the webiste as follows:

“Duties are not limited to, but will include:
– Conduct research to support the activities of the Center.
– Manage program contacts and knowledge.
– Draft correspondence and take notes at Council roundtables and strategy sessions for program archives.
– Handle logistical issues for program events and activities.
– Help draft policy briefs and other Center publications.
– Follow current events in one region or country and produce a daily briefing.
– Develop and maintain contacts for the Center.”

Am I supposed to write something specific for this or just attach a former essay that I’ve written? How long should it be?