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.