To calculate the number of days between two dates in Oracle SQL, you can simply subtract the earlier date from the later date, and Oracle will return the number of days between them.
Option A uses SYSDATE to get the current date and TO_DATE to convert the string '01-JANUARY-2019' to a date using the default date format, which matches 'DD-MON-RR'. The subtraction here will correctly give the number of days between these two dates.
Option C is similar to A but includes the ROUND function, which is used to round the number to the nearest whole number. Since the difference between two dates in Oracle SQL is a decimal value representing the number of days and fractional days, rounding will give us the whole number of days, which is what we want.
Options B, D, and E are incorrect because they either use an invalid date format (the default format is 'DD-MON-RR', not 'DD/MONTH/YYYY' or 'DD-MON-YYYY'), perform invalid operations (such as subtracting a string from a date), or misuse SYSDATE.