The correct option is
A I think I'd rather have a blue one.
The phrase, 'to look for something' means 'to search for something' or '
to try to find something that one has lost'. In the given sentence, the salesperson responds to Sandy by saying, "Let me look for it'. This means that he wants to search for something which Sandy has asked.The salesperson also says, 'Would you like to try it'. The phrase, 'to try something on' means 'to put on an item of clothing to see if it fits or suits one'. Hence the word, 'it' refers to a piece of clothing which Sandy has asked for in the previous sentence.
Option A: This is an affirmative sentence, which conveys that Sandy does not prefer white jackets. He has not asked for a piece of clothing in this sentence. Option A is incorrect.
Option B: In this sentence, Sandy has expressed his preference for a particular piece of clothing in blue colour. In response, the salesperson mentions that he will search for it. This sentence in option B completes the conversation meaningfully. Hence, option B is correct.
Options C and D: The questions, 'How much is it?' and 'Can I try it?' indicate that Sandy is referring to something which is available in front of them. For such a question, the salesperson would not respond that he would search for it. Hence, options C and D do not complete the conversation meaningfully and are incorrect.