The correct option is C think of something
The person in the conversation, Jai, expresses concern because he does not know how to complete his project on time.
In an attempt to console him, Rita reassures him, telling him not to worry.
Option C: This line completes the conversation in meaning, 'think of something' refers to how Jai will think of something in order to complete his project.
Hence option C is correct.
The rest of the options, 'kill the time', 'need some help' and 'find it out' are vague and do not complete the sentence in meaning. Also, they are not words of reassurance.
Hence options A, B and D are incorrect.