Step-by-step Solution:

The phrase out of the blue means something that happens suddenly or unexpectedly, without any prior warning. Let us analyze the options:\[\] (a) stranger - This is incorrect as it is unrelated to the idiom's meaning.\[\] (b) unexpectedly - This is the correct meaning, as it aligns perfectly with the phrase.\[\] (c) to bring happiness - This is incorrect; the idiom does not imply happiness but rather an element of surprise.\[\] (d) emerging out of blue colour - This is a literal interpretation and does not convey the idiomatic meaning. \[\] \[\] Correct Answer: (b)