def get_lsf_grid_name(fibre_number):
    """
    Return the appropriate LSF name (a, b, c, or d) to use, given a mean fiber number.

    :param fiber_number:
        The mean fiber number of observations.
    
    :returns:
        A one-length string describing which LSF grid to use ('a', 'b', 'c', or 'd').
    """
    if 50 >= fibre_number >= 1:
        return "d"
    if 145 >= fibre_number > 50:
        return "c"
    if 245 >= fibre_number > 145:
        return "b"
    if 300 >= fibre_number > 245:
        return "a"