1) Re-read the letter; FTB usually includes further details/options on response expected.

2) Call the contact number provided within the letter.

3) Your written response should include information validating your presence outside of CA throughout  2015. This could include your 2015 tax return from other State(s)/country, W-2 and/or 1099 forms mailed to non-CA address, and utility bills or rent agreement for your non-CA address.

Even though your residence was outside of California in 2015, if you had any CA sourced income in 2015, you may have needed to file a CA non-resident income tax return.  CA sourced income could be such items as  a CA rental or ownership in a CA based business.