(A) goes in a direction opposite to the author’s position. The author assumes that if foreign travel had been less expensive 30 years ago, British people would have been more likely to take vacations abroad. It doesn’t make sense for the author to assume that British people still wouldn’t have had enough money to vacation abroad, because the author believes that expense of travel is what was making them less likely to travel before. Reduced expense would, in the author’s mind, make foreign travel more likely. In any case, the argument is about comparative proportions of people who vacation abroad. That doesn’t require an assumption that British people, as a whole, would have had enough money or would not have had enough money to travel abroad.

The argument doesn’t concern people outside Britain traveling to Britain for vacation.

Not necessary, because the author doesn’t have to have an opinion about the domestic vacation practices of British people. We know a smaller % of the British people traveled abroad for vacations 30 years ago. That doesn’t imply that greater % than today traveled domestically; perhaps people just took fewer vacations generally 30 years ago.

Necessary, because if it were not true — if more British people of 30 years ago would NOT have traveled abroad even if they had enough money to vacation abroad — that suggests the true explanation for the lower % who traveled abroad 30 years ago wasn’t a lack of money to spend on vacations. It raises the possibility that there were other factors leading to lower foreign travel — perhaps cultural issues or conflict in foreign countries, etc.

Not necessary, because the argument concludes merely that British people have “more money to spend on vacations now” compared to 30 years ago. This doesn’t commit the author to an opinion about what occurs if British people are “wealthier.” There’s a difference beteween having more money available for vacations and being wealthier. Someone can have more money available for vacations because housing expenses or food expenses have declined; this doesn’t necessarily imply someone is “wealthier.”