Exercise about voltage thresholds of logical port
Since it takes time to change the voltage, the minimum transition time will occur when your signal has to make the smallest possible change in voltage. If the signal starts as a logic '0' then its maximum voltage is \$V_{IL}\$. The smallest voltage that will be recognized as a logic '1' is \$V_{IH}\$. So, the smallest possible voltage change from logic low to logic high is from \$V_{IL}\$ to \$V_{IH}\$ and the minimum transition time is the time required for the signal voltage to make that change.