The function ADD_MONTHS(hire_date, 6) adds 6 months to the hire_date. The function NEXT_DAY(date, 'day_name') finds the date of the first specified day_name after the date given. In this case, 'MONDAY' is used to find the date of the first Monday after the hire_date plus 6 months.
Option A is correct as it accurately composes both ADD_MONTHS and NEXT_DAY functions to fulfill the requirement.
Options B, C, and D do not provide a valid use of the NEXT_DAY function, either because of incorrect syntax or incorrect logic in calculating the required date.
The Oracle Database SQL Language Reference for 12c specifies how these date functions should be used.