The correct option is C has been suffering
Here, 'has been ....' in present continuous is correct it
implies an action going on at the time of speaking.
'Will be ...' in future continuous tense
is incorrect because it indicates what will be going on in the near future.
'Is suffering' in present simple tense
is correct because it is used to indicate a future event on a designated
date as part of a plan.
'Was suffering' in past continuous incorrect
because it is used to indicate uncompleted action in the past (with or without
time reference).