group companies rollover releif

this is a qtn from kaplan kit

fogo ltd owns 100% subsidiary netta ltd. fogo sold a warehouse nd this gain is rolled over into purchase of fixed plant nd machinery by netta ltd select which f the followng will not result in the gain becoming chargeable:
a)netta lts selling the fixed plant nd machinery
b)fogo selling the shareholding in netta ltd
c)netta ceasing to use the machine in trade
d)10 yrs since purchase of machinery

can u explain the ans i didnt understand the explanaion they gave