The correct option is
D nick
The given sentence means that" 'The little child would have drowned. However, he or she did not because the lifeguard came in just in time and saved the child".
The phrase, 'nick of time' means 'at the last moment' or 'just in time'. The word, 'nick' meaningfully fills the blank as it conveys that the lifeguard came in just in time. Hence, option C is correct.
The word, 'ring' is a noun, which refers to a circular object. It is also a verb which means 'make a sound'. Another meaning of the verb is 'surround (someone or something), especially for protection or containment'. Given these meanings, the word, 'ring' does not fill the blank meaningfully. Hence, option A is incorrect.
The phrase, 'right of time' conveys no meaning. A more meaningful phrase using the word 'right' would be '... the lifeguard who came in at the right time'. Hence, option B is incorrect.
Option D is incorrect as option C is the correct answer.