"She looks pretty with that dress."
Translation:Elle est jolie avec cette robe.
The demonstrative adjectives are ce, cet, cette, ces
ce = singular masculine
cet = singular masculine, but the following word has a vowel or vowel sound (primarily 'h'). par example: cet homme
cette = singular feminine
ces = plural
In this case it's cette robe because robe is feminine. Keep in mind that cet and cette sound the same, so you will need to know the gender of the noun in order to know which one to use.
EDIT: I just noticed a good explanation above of the same rules by sitesurf.
One of the correct answers is Elle est belle dans cette robe. My answer was Elle est jolie dans cette robe and it wasn't accepted! Both adjectives are correct and the preposition (what is actually tested here as I'm trying to test out of it) is the same. This is getting to be REALLY annoying!
"Elle a l'air jolie avec cette robe"
This answer was accepted, but it seems unusual that the verb être is marked as a correct answer. I was always taught that when someone has an opinion or a "look" to them it's the expression avoir + l'air. She seems/looks happy => "Elle a l'air contente". I'm not a native speaker, but I wouldn't use "être" in this case.
All three are demonstrative adjectives:
ce: in front of a masculine singular noun starting with a consonant: ce livre
cet: in front of a masculine singular noun starting with a vowel or a non aspirate H: cet ami, cet homme
cette: in front of a feminine singular noun: cette femme
ces: in front of a plural noun: ces livres, ces amis, ces hommes, ces femmes.