Option B (1998) is the correct answer. The textbook context explicitly states that 'The year 1998 was the warmest year, probably not only for the 20th century but also for the whole millennium.' Additionally, the context mentions that 'The seven warmest years during the 1856-2000 were recorded in the last decade,' and the 1990s decade 'recorded the warmest temperature of the century.' The other options (1990, 1885, 1950) are not mentioned as record-warm years in the provided context.