Coherent, functional, and professional communication is integral to the proper functioning of any business. Most companies are interested in hiring candidates who can clearly and effectively communicate across all forms. Everyone has different communication styles and preferences, depending on the person. By asking for your preferred method of communication, they're gauging your aptitude towards every method of communication, verbal or written. Knowing this can provide your interviewer with insights on how to place you within their teams.

Best to strike a balance of above average writing skills and above average oral communication skills. Communication is more than getting everyone on the same page. It's about how a message is conveyed. Take care with your choice of words and what they might communicate.

If this comes as the last question, they might simply be asking how to get a hold of you, in which case you should simply state your preference out of the contact information you've provided on your resume and cover letter. Best to clarify.