Final answer:
The word 'mega' is best synonymous with 'large,' which is option 3 in the given choices.
Step-by-step explanation:
The word 'Mega' is synonymous with things that are extremely large or great in scale. Among the listed choices, the most appropriate synonym would be option 3, 'Large'. 'Ton' often refers to a specific measure of weight -- one metric ton is equivalent to 1,000 kilograms, which is slightly larger than the Imperial 'short ton'. However, 'ton' in this context is not a direct synonym for 'mega.' Options like 'sangfroid' and 'big' do not offer the same magnitude implied by 'mega,' with 'sangfroid' not relating to size at all and 'big' being more comparable but not as emphatic as 'mega' or 'large.'