use the following custom method
-(NSArray *)stringArrayFromJsonFile:(NSString *)jsonFileName withKey:(NSString *)key
{
    NSData *fileContents = [NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:jsonFileName ofType:@"json"]];
    NSError *error;
    NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:fileContents options:kNilOptions error:&error];
    NSArray * stringArray = [dict objectForKey:key];
    return stringArray;
}
now call that method like
NSArray* stringArray =  [self stringArrayFromJsonFile:@"yourJsonFileName" withKey:@"categories"];
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solved How to Parse an Array of Strings (JSON) – iOS?