A: You need to add an 'is' to A for it to work, because "The solution by far the best" doesn't make sense. The sentence should be "The solution is by far the best". This would make it: "The solution, which is to have a day at work where people do not use computers, is in my opinion, by far the best." B: B, which in my opinion is by far the best option, is correct only if you make some adjustments (see what I did there). 'Computer' should be plural and one comma needs to be removed. It also looks like you deleted 'a day' by accident. "The solution, which in my opinion is by far the best, is to have A DAY at work where people do not use computerS" is the best option in my opinion. After the corrections, I'd choose B.

B is absolutely not ok. in my opinion 不是用来形容这个solution的。a倒是还可以。但是定语太长一般不直接跟在后面，容易引起歧义，可以写成这样： The solution, in my opinion, by far the best, which is to have a day at work where people do not use computers, 这样稍微好点。你的原句也是可以的。这句不会引起歧义。

B is absolutely not ok. in my opinion 不是用来形容这个solution的。a倒是还可以。但是定语太长一般不直接跟在后面，容易引起歧义，可以写成这样： The solution, in my opinion, by far the best, which is to have a day at work where people do not use computers, 这样稍微好点。你的原句也是可以的。这句不会引起歧义。