def biggest(aDict):
    '''
    aDict: A dictionary, where all the values are lists.

    returns: The key with the largest number of values associated with it
    '''

    return max(aDict, key=lambda x: len(aDict[x]))

animals = { 'a': ['aardvark'], 'b': ['baboon'], 'c': ['coati']}

animals['d'] = ['donkey']
animals['d'].append('dog')
animals['d'].append('dingo')

print(biggest(animals))