In Oracle SQL, to include single quotes in a string literal, you need to use two single-quote characters.
A . This statement will not run successfully because the single quotes are not properly escaped. After the text The first_name is , there should be two single quotes to start the string literal for the first name.
B . This statement will run successfully. It properly escapes the single quotes by using two single quotes around first_name. This will result in the output like The first_name is 'John'.
C . This statement will run successfully as well. The two single quotes after The first_name is and before FROM employees are correctly used to delimit the first name.
D . This statement will not run successfully. There is a missing single quote before the double pipe (||) operator, and at the end of the statement, only one single quote is used instead of two for escaping.
E . This statement will not run successfully because the backslash is not used to escape single quotes in Oracle SQL. The proper method is to use two single quotes without a backslash.
Reference:
Oracle Database SQL Language Reference, 12c Release 1 (12.1): "Text Literals" Oracle Database SQL Language Reference, 12c Release 1 (12.1): "String Concatenation Operator"