As per section 224(6)
The Board my fill any casual vacancy in the office of an auditor;... Provided that where such vacancy caused by the resignation of an auditor, the vacancy shall only be filled by the company in general meeting (AGM or EGM).
Casual Vacancy:-
The expression casual vacancy has not been defined in the Companies Act, 1956. Simply stated, a casual vacancy in the office of an auditor means a vacancy caused in the office of an auditor by his death, disqualification, resignation etc.
It has been held in the case of the Institute of Chartered Accountants of India vs. Jnanendranath Saikia that casual vacancy is not a vacancy created by any deliberate ommission on the part of the company to appoint an auditor at its annual general meeting.
Where the casual vacancy is caused by resignation of an auditor, the Board cannot fill up the casual vacancy but place the matter before the company in the General Meeting for appointing an auditor in respect of the casual vacancy.
The resolution would ofcouse be Ordinary Resolution.