As of 2021-05-01, the southern end of "Trail D" is very aggressively marked "trail closed, private property, no trespassing". No markings were visible looking southeast from the Straightback Mtn Trail / Jesus Valley Beaver Pond Trail junction.

I suspect it's only the first ~200 meter "Trail D" segment, see Alton map 14, block 1, lot 2: https://www.alton.nh.gov/maps/Alton14.pdf
Different owner from map 14 block 1 lot 1 to the north: https://www.alton.nh.gov/forms/assessing/Alton%20Assessment%20Information%20-%20Map%20&%20Lot.pdf