Cocoa-Touch - How parse local Json file

JSON has a strict key/Value notation, your key/value pairs for R4 and response are not correct. Try this:

NSString *jsonString = @"{\"quizz\":[{\"id\":\"1\",\"Q1\":\"When Mickey was born\",\"R1\":\"1920\",\"R2\":\"1965\",\"R3\":\"1923\",\"R4\":\"1234\",\"response\":\"1920\"}]}";

^{If you read the string from a file, you don't need all the slashes}
Your file would be something like this:

{"quizz":[{"id":"1","Q1":"When Mickey was born","R1":"1920","R2":"1965","R3":"1923","R4":"1234","response":"1920"},{"id":"1","Q1":"When start the cold war","R1":"1920","R2":"1965","R3":"1923","R4":"1234","reponse":"1920"}]}

I tested with this code:

NSString *jsonString = @"{\"quizz\":[{\"id\":\"1\",\"Q1\":\"When Mickey was born\",\"R1\":\"1920\",\"R2\":\"1965\",\"R3\":\"1923\",\"R4\":\"1234\",\"response\":\"1920\"}, {\"id\":\"1\",\"Q1\":\"When start the cold war\",\"R1\":\"1920\",\"R2\":\"1965\",\"R3\":\"1923\",\"R4\":\"1234\",\"reponse\":\"1920\"}]}";NSLog(@"%@", jsonString);NSError *error =  nil;NSDictionary *json = [NSJSONSerialization JSONObjectWithData:[jsonString dataUsingEncoding:NSUTF8StringEncoding] options:kNilOptions error:&error];NSArray *items = [json valueForKeyPath:@"quizz"];NSEnumerator *enumerator = [items objectEnumerator];NSDictionary* item;while (item = (NSDictionary*)[enumerator nextObject]) {    NSLog(@"clientId = %@",  [item objectForKey:@"id"]);    NSLog(@"clientName = %@",[item objectForKey:@"Q1"]);    NSLog(@"job = %@",       [item objectForKey:@"Q2"]);}

I got the impression, that you copied old code, as you are not using apple's serialization and a Enumerator instead of Fast Enumeration. The whole enumeration stuff could be written simple as

NSArray *items = [json valueForKeyPath:@"quizz"];for (NSDictionary *item in items) {    NSLog(@"clientId = %@",  [item objectForKey:@"id"]);    NSLog(@"clientName = %@",[item objectForKey:@"Q1"]);    NSLog(@"job = %@",       [item objectForKey:@"Q2"]);}

or even fancier with block based enumeration, hwere you have additionaly an index if needed to the fast and secure enumeration.

NSArray *items = [json valueForKeyPath:@"quizz"];[items enumerateObjectsUsingBlock:^(NSDictionary *item , NSUInteger idx, BOOL *stop) {    NSLog(@"clientId = %@",  [item objectForKey:@"id"]);    NSLog(@"clientName = %@",[item objectForKey:@"Q1"]);    NSLog(@"job = %@",       [item objectForKey:@"Q2"]);}];

NSString *filePath = [[NSBundle mainBundle] pathForResource:@"fileName" ofType:@"json"];NSString *myJSON = [[NSString alloc] initWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:NULL];NSError *error =  nil;NSArray *jsonDataArray = [NSJSONSerialization JSONObjectWithData:[myJSON dataUsingEncoding:NSUTF8StringEncoding] options:kNilOptions error:&error];

Use jsonlint.com to find errors in your JSON string.
In this case, it says you have non-valid JSON near "R4"