Correct Answer: out of the blue

Solution : The correct choice is the t hird option.

Out of the blue means something happening unexpectedly, without any warning or anticipation.

The meanings of the other options are as follows:

• Blowing off steam means to express or release one's pent-up emotions, frustrations, or stress in a loud or vigorous manner.
• Like a cakewalk suggests, something is very easy to do. It implies that a task or situation is simple and requires little effort.
• Stepping up his game means making an effort to improve or enhance one's performance, skills, or abilities.

Therefore, the correct sentence is: "Her son came from Europe to meet her out of the blue after 20 years."