In the given sentence, the following is conveyed: 'As far as one can see, it appears that the two countries are talking peace. However, both the countries are appearing to talk peace only for the sake of putting up a show'.
The idiom, 'on the face of it' means 'without knowing all of the relevant facts'. It also means 'apparently' or 'as far as one can see'. Given this meaning, this idiom meaningfully completes the sentence. Hence, option B is correct.
Option A: 'To tell one to face' is incomplete and not meaningful in itself. Hence, option A is incorrect.
Options C & D: 'To lose face' means 'to be humiliated or come to be less highly respected'. 'In the face of' means 'when confronted with'. Given these meanings, these idioms cannot meaningfully complete the given sentence. Hence, options C and D are incorrect.