To answer this question, let's understand the given regular expression pattern and its usage in the code:
s/(\w)/\u$1/g;
This regular expression pattern is used to replace every occurrence of a lowercase letter with its uppercase version in a string.
Explanation for each option:
Option A) Changes all lowercase letters in the string stored in the variable $_ to upper case - This option is correct because the given regular expression code will change all lowercase letters to uppercase in the string.
Option B) Changes the first letter in the string stored in the variable $_ to upper case - This option is incorrect because the given regular expression code will change all lowercase letters to uppercase, not just the first letter.
Option C) Changes all letters in the string stored in the most recently used scalar variable to upper case - This option is incorrect because the regular expression code does not refer to a specific variable. It operates on the default variable $_.
Option D) Adds \u in front of every letter or digit in the string in $_ - This option is incorrect because the regular expression code does not add \u in front of every letter or digit. It only changes lowercase letters to uppercase.
Therefore, the correct answer is A) Changes all lowercase letters in the string stored in the variable $_ to upper case.