`\text{a)}`
`2^27 = (2^3)^9 = 8^9`
`3^18 = (3^2)^9 = 9^9`
`-> 9^9 > 8^9`
`-> 3^18 > 2^27`
$\\$
`\text{b)}`
`2^91 = (2^13)^7 = 8192^7`
`5^35 = (5^5)^7 = 3125^7`
`-> 8192^7 > 3125^7`
`-> 2^91 > 5^35`
$\\$
`\text{c)}`
`2^225 = ( 2^15)^15 =32768^15`
`3^150 = (3^10)^15 = 5949^15`
`-> 32768^15 > 5949^15`
`-> 2^225 > 3^150`
$\\$
`9^12 = (3^2)^12 = 3^24`
`27^7 = (3^3)^7 =3^21`
`-> 3^24 > 3^21`
`-> 9^12 > 27^7`