The correct option is B Nobody
The word 'punctual' means 'to be on time'. It's the opposite of 'late' (to arrive later than a fixed time).
Option C: 'Nobody' means 'no person'. It agrees with the sentence because 'all the children' being punctual means that 'none of them was late'. Hence option C is correct.
Option A: 'Anybody' means 'any person, it does not matter who'. This word does not complete the sentence in meaning.
Option B: 'Everybody' means 'all people'. It refers to all the children: if all the children were on time 'punctual', they cannot be 'late'.
Option D: 'Somebody' means some person. As it's mentioned that 'all' of the children were on time, it isn't logical to say that some one was late.
Hence options A, B and D are incorrect.