From the sentences a, b, c and d, sentence 'b' sounds introductory. It begins the topic, that the person's friend selected everything quickly. This is a general statement, which is then expanded by the other sentences that follow.
Hence, 'b' comes first.
Sentence 'c' follows 'b' as the word 'selection' in 'c' refers to the words 'selected everything he needed' in 'b'.
Sentence 'a' follows 'c' as this question is in response to the first two statements. The question is asked by a second person.
Sentence 'd' follows 'a' as this statement is an answer to the question asked in 'a'. The answer is given by the first person who started the topic.
Hence, the correct sequence is: b, c, a, d. This can be seen in option C, hence it is correct.
Options B and D: Sentence 'a' cannot come first, because it asks a question that isn't introductory or does not seem to start the topic by itself. The words 'so quickly' mean that the topic has already been started. Hence 'a' cannot come first in the sequence. So, options B and D are incorrect.
Option A: Similarly, sentence 'c' also cannot come first, because 'but' suggests that it is a continuation of some topic. Hence option A is incorrect.