Which of these inequalities means that
18 is greater than three times a number?

Let's write this verbal phrase as an inequality.
First of all, let the number be n.
"three times n" can be written like so:-
[tex]\pmb{3n}[/tex]
Now, 18 is greater than 3n:-
[tex]\pmb{18 > 3n}[/tex]
Which means 3n is less than 18:-
[tex]\bigstar{\boxed{\pmb{3n < 18}}}[/tex]
Hope everything is clear; if you need any explanation/clarification, kindly let me know, and I'll comment and/or edit my answer :)