what do they mean? "impeccably attired" "painfully obvious" "perfectly plain"

[japanjapan]

Dear teachers,
I did a collocation exercise today. and the following is the answer.
impeccably attired
painfully obvious
perfectly plain
now look at the first, attired, does it mean clothes, is its meaning well-dressed?
painfully obvious, what's the meaning painfully? can you give me a sentence to illustrate its meaning?
perfectly plain, what's the meaning of plain? please give me example sentence.

thanks a lot,

by the way,
please look at the following sentences:
1, In what country was he born?
2, What country was he born in?
these two sentences, which one is formal? which one is informal? why?

thank you again.

[mykwyner]

impeccably attired...very well dressed

painfully obvious...so obvious that it would cause pain if it could

perfectly plain...completely unadorned, not fancy in any way

It is less formal to end a sentence with a preposition (like in).