From a few quick calculations (as I do not have a whizz wheel with me), I get close to a).
Pressure altitude for the high ground is 6,780ft, which gives an ISA deviation of -15 if you round to the nearest degree. A rule of thumb for the extent of temperature error is 4% per degree of ISA deviation per thousand feet of indicated altitude.
4 x (-15) x 7 = -420ft
The true altitude is thus 420ft lower than indicated, so terrain clearance is:
6,580ft (true alt) - 6,420ft (elev) = approximately 160 ft