For the EMPLOYEES table and given that NLS_DATE_FORMAT is set to DD-MON-YY, the query that requires explicit data type conversion is:
D . SELECT join_date FROM employees where join_date > '10-02-2018'; This query compares the JOIN_DATE column of type DATE with a string '10-02-2018'. The string must be explicitly converted to a DATE using the TO_DATE function with a format mask that matches the NLS_DATE_FORMAT or the string literal. Options A, B, C, and E do not necessarily require explicit data type conversion: A does not require conversion because adding a number to a number column is a valid operation. B is syntactically incorrect; the correct function to extract a substring is SUBSTR(join_date, 1, 2) and that would require an explicit conversion since JOIN_DATE is a DATE and SUBSTR expects a string, but it seems to be a typo. C uses an invalid concatenation operation with the pipe symbols, which could be a typo or misunderstanding, but would not typically require a data type conversion. E requires implicit data type conversion because adding a number to a date is valid in Oracle, which adds the number as days to the date.
Reference: Oracle Database SQL Language Reference, 12c Release 1 (12.1)