NSError *error = [languageModelGenerator
                    generateRejectingLanguageModelFromArray:languageArray
                    withFilesNamed:filesName
                      withOptionalExclusions:@[@"b", @"c", @"d", @"f", @"g", @"k", @"l", @"n", @"p", @"r"]
                    usingVowelsOnly:FALSE
                    withWeight:nil
                    forAcousticModelAtPath:[AcousticModel
                                            pathToModel:@"AcousticModelEnglish"]]; // Change "AcousticModelEnglish"
    
    [languageModelGenerator deliverRejectedSpeechInHypotheses:YES];

I have never received rejected speech hypotheses.
I have tried to add optional exclusions but I am not sure I am doing write.
My problem is how to make the voice recognition for those words as precise as possible. I mean if user says “NOO” instead of “MOO”, or “PEOW” instead of “MEOW”, I want “NOO” and “PEOW” to be skipped.

Couple of things that may help. Here is the description of how the optional exclusions work:

optionalExclusions can either be set to nil, or given an NSArray of NSStrings which contain phonemes which you do not want to have added to the rejection model. A case in which you might want to do this is when you have over-active rejection such that words that are really in the vocabulary are being rejected. You can first turn on deliverRejectedSpeechInHypotheses: in order to see which phonemes are being detected overzealously and then you can add them to the exclusionArray. Set this parameter to nil if you aren’t using it.

So issue one is that you have to give it phonemes instead of letters, but issue two is that you only use it if you already have a set of phonemes that you’ve verified are being rejected overzealously via the use of the deliverRejectedSpeechInHypotheses setting. It isn’t something you want to start out with.

You haven’t mentioned whether MEOW and MOO are being recognized when they are being said correctly, so let’s start there and then see if I can think of some suggestions for refining things.

Yes “MEOW” and “MOO” are being recognized when they are being said correctly.
But the problem is that if I say “POO”, “TEOW”, “TOO” or “PEOW”; they are recognized too. I have set deliverRejectedSpeechInHypotheses to YES.

To me, it doesn’t sound a highly-likely usage scenario since your app is probably more designed for users to say “meow” to it than to intentionally interact with it by saying “peow” with the expectation that “peow” isn’t recognized. But probably the most effective way you have available to prevent recognition of “peow” is to add “peow” to your main language model and then ignore it when it is recognized. Does that make sense?