As the number of employees is prime we can add the numerator and denominator of ratios directly to find the number of employees.
1. 101 + 88 = 189, which is not a prime number.
2. 87 + 100 = 187, which is not a prime number.
3. 110 + 111 = 221, which is not a prime number.
4. 85 + 98 = 183, which is not a prime number.
5. 97 + 84 = 181, which is a prime number.
The ratio of employees = 97:84
Hence, option (5)97:84 .is the correct choice.