1. 오랜만에 : after a long time (1) 오랜만에 집에서 쉬니까 너무 심심해요. Answer : (B)I feel boring because it has been a long time since last time I rested at home. (2)오랜만에 이곳에 왔어요 Answer : (B)It has been a long time since last time I came here. 2. 오랜만에 & ~만에 1) 오랜만에 : This is just a phrase that means 'after a long period of time'. It once meant something related to '~만에' but now it is just a phrase. So please think it as the meaning itself : 'after a long time'. Also, it is working as an adverb inside a sentence. 2) ~만에 - Meaning : after a certain time has passed - Using inside a sentence : ~만에 + action => Meaning of ~만에 + action : after a certain time has passed, the 'action' happened => So, ~만에 doesn't mean 'something happened a after period of time' it is more closer to the meaning of 'after a certain time has passed, a certain action happened'. I think this is where you got confused. => Also, if you want to use '~만에', the same thing should not be happened during those times. So for example... Ex) 3년 만에 집에서 쉬었어요. (I rested at home after 3 years.) -> 3년 + ~만에 + 집에서 쉬었어요. (action) 1. This sentence specifically means : after 3 years has passed, I rest at home. 2. This also shows that you had never taken any rest at home during those 3 years, not even once. After 3 years you could 'finally' rest at home. Another example... Ex) 2달 만에 오빠를 만났어요. (I met my older broter after 2 months.) -> 2달 + ~만에 + 오빠를 만났어요. (action) 1. This sentence specifically means : after 2 months has passed, I met my older brother. 2. This also shows that you had never met your brother for last 2 months, not even once. After 2 months you could 'finally' meet your older brother. So.. ~만에 implies two things at the same time. 1. After a certain time has passed, the action happened. 2. Same action did not happened for that certain amount of time. Hope it can be a help~! : )

Yeah I was totally confused when I saw the first example sentence on my book, so I guess it's not my problem haha! Thank you for showing me a better phrase of 오랫동안 to replace the original one. Really learned a thing!

The first example sentence is a terrible one which is more confusing than clarifying anything. More logical example would 오랜만에 집에서 쉬니까 너무 좋아요, or 너무 오랫동안 쉬니까 심심해요. (오랫동안 means doing something continuously for a long time, almost the opposite of 오랜만에)