The correct answers are:

Informal organisation can stifle innovation

and

Managers in positions of authority generally cannot be part of the informal organisation

'The influence of the informal organisation was highlighted by the Hawthorne Studies, in the way group norms and dynamics affected productivity' is a fact.

'Informal organisation can pose a threat to employee health and safety' is true, because of informal 'short cuts' which are often developed and shared, by-passing health and safety rules and procedures.

'Informal organisation can stifle innovation' is not true, because the 'grapevine' encourages knowledge sharing and multi-directional communication.

'Managers in positions of authority generally cannot be part of the informal organisation' is not true: managers can feed information into the grapevine and be part of their own informal networks.