(a) below 10c with visible moisture
(b) de-icing is done re frost/snow/ freezing rain etc and even if the temp is above freezing, falling snow grains etc require de-ice ....commaders decision here.
(c) yes for flex but not allowed with a contaminated rwy
(d) 10c and visibile moisture upto -40 sat in climb and Crz. In Des visible moisture , ice on irrespective of temp colder than -40 sat.
jeez I feel I have done a tech exam.