Answer: A) Cross-object fields update between the project and account records are not supported.
B) The account lookup field on the project record can be made optional
Explanation: For maintaining the relation between the client's account record and project object , the private sharing model will work as permitting the access for the additional object such as tracking , searching and updating the records of both the sides.
Account lookup field option for looking for the run time of the object , search criteria of the object etc can be kept as optional category.
Other options are not appropriately related with the situation mentioned in the question because users cannot delete the records and roll-up summary cannot be present with project record's data.Thus, the correct options are option(a) and (b).