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Baby Boomers are affluent BECAUSE they've already set up their lives and sent thier children out of the nest. That eliminates (D) and (E) which both incorrectly introduce a contrast connector.
From there, it's a matter of parallelism. 'Having' isn't a verb in itself, but rather an auxiliary verb; 'having' and 'seeing' aren't parallel in (B). (C) doesn't make any sense; "Having...their children being seen off" is terrible at a glance.