What is indicated by the following report? TYR UUA/OV TYR180015/TM 1757/FL310/TP B737/TB MOD-SEV CAT 350-390
a) Thunderstorm activity at FL310
b) Severe turbulence encountered between 35,000 and 39,000 feet
c) Moderate turbulence encountered between 35,000 and 39,000 feet
d) Clear air turbulence between 350 and 390 degrees