If you have a secure password, then it is very difficult for a hacker to randomly guess your password. Is it safer to have an 8-character password that can contain any character {a-z, A-Z, 0-9, !@#$%^&*() } or a 12-character password with only lower-case letters?
Take a second and make a guess as to which is more secure. Then use the Fundamental Counting Rule to find the number of possible 8-character and 12-character passwords. The option with more possible passwords would be more secure as it is more difficult to randomly guess.
For the 8-character complex password, there are 26 letters (lower- and upper-case), 10 digits, and 10 special characters. That is a total of 72 different characters. Each character of the password can be any of the 72 characters, so the number of different possible passwords is,
\[\begin{align*} 72 * 72 * 72 * 72 * 72 * 72 * 72 * 72 &= 72^8 \\ &= 7.222*10^{14} \\ &= 722,200,000,000,000 (722.2 trillion) \end{align*}\]For the 12-character simple password, there are only 26 different possible characters. Each character can be any of the 26 characters, so the number of different possible passwords is,
\[\begin{align*} 26 * 26 * 26 * 26 * 26 * 26 * 26 * 26 * 26 * 26 * 26 * 26 &= 26^12 \\ &= 9.543*10^{16} \\ &= 95,430,000,000,000,000 (95.4 quadrillion) \end{align*}\]Since there are more possible passwords for the 12-character simple password, this would be more secure password.
Note: Of course, length combined with complexity is a better option, so the more secure option would be a 12-character password with any character {a-z, A-Z, 0-9, !@#$%^&*() }. Passphrases that are 15-30 characters long are even more secure. However, these options weren’t addressed in the question.