Cutting and pasting so people don't need to go back up to read the choices.

] 1. She knows the Ferrars well, likes them and does not mind marrying either Robert or Edward.
] 2. She has just heard of the Ferrars, and does not mind marrying either Robert or Edward. She will obey any order given to her.
] 3. She knows the Ferrars, hates them and does not want to marry either Robert or Edward. But her family lets the Ferrars believe that Edward (or Robert) may marry her, although it is quite unlikely.
] 4. She has never heard of the Ferrars, and can be ready for a big surprise.
] 5. Miss Morton does not exist. She is just a product of Mrs Ferrars' wishful imagination.

I'm inclined, like Claire, to go with 2 or 4, though 5 could be a factor in one way (I'm sure Miss Morton exists - even Robert the not-so-sharp would start wondering why he'd never even heard of the daughter of a Lord Morton). For the rest -

1) If Miss Morton knew the Ferrarses at all well, both of the young men would have met her several times. But neither Edward nor Robert nor anyone makes any mention of a meeting, and if they had met several times even the lizard-hearted Ferrars crowd would have eager to see in her a preference for one young man or the other (look! She's getting attached!) even if no such preference existed.

3) If she hated and refused to marry either of them, her family, while it might be quite exasperated with her, would have no reason to go on dangling Mrs. Ferrars like that. Granted, it would continue to keep them in her good graces if she didn't know, but I get the impression (since her father is Lord Morton - gasp, drumroll) that the Ferrarses are more conscious of receiving honour than of bestowing it. Lord and Lady Morton can likely go everywhere the Ferrarses can, and more - cultivating Mrs. Ferrars on a false pretext wouldn't get them anything except possibly some nasty gossip about their daughter later on.

2 and 4 both seem likely; for 2, while we don't know that she's as docile as all that, it's conceivable that she's heard of the Ferrarses and doesn't think much about them one way or the other - lots of eligible young men must have been leaving their cards for her and her 30,000 pounds. For 4 - yes, it's easy to see the parents negotiating something like this while not understanding any necessity to take her into their confidence. Not that they could engage her or anything, but once things were agreed on with Mrs. Ferrars, they could take any and every opportunity to shove the prospective bridegroom-elect down her throat.

For 5: While I'm sure Miss Morton exists, I'm not so sure that her parents are as eager to ally with the Ferrarses as Mrs. Ferrars is to ally with them. While Edward, and then Robert, has the prospect of a large sum of money, Miss Morton is definitely up there - not a Duke's daughter or anything like that, but moneyed enough that she could marry pretty much anyone. Mrs. Ferrars might have had a polite conversation with Lady Morton one day and managed to transfer that into some sort of realistic possibility of her wanting her daughter to marry one of Mrs. Ferrars's sons. So while Miss-Morton-as-eligible-match exists, Miss-Morton-as-realistic-Ferrars-bride may not.