Answer: A.
Step-by-step explanation:
You multiply the constants (2 and 4), then multiply the variables. To multiply variables raised to a power, you keep the variable (k) the same, and add the exponents. 2 x 4 =6....... k^4 x k^1 = k^5
5: 90 can also be written as a fraction 5/90 then reduce the fraction
5/90 (5*1=5/5*18=90) so it reduces to 1/18
the ratio simplified is 1:18