If the auditor is unable to take information regarding some material things from management even after discussing matter with BOD then what kind of opinion will be expressed by the auditor in audit report?

Management is not actually the topmost alternative source of audit evidence and information as the management is working in favor of the entity, thus, information from them tend to be subjective and biased. In this respect, if there is an information needed and the management cannot supply the said information, still, there exist external sources of evidence and information which can provide you with more reliable and objective evidence. If you are thinking of disclaiming an opinion just because you weren't able to seek information from the management, think of the topmost source of evidence - external parties. Now, if after exhausting all the efforts and alternatives in obtaining such evidence yet, you cannot obtain any evidence and information to support the material balances and underlying details, that is the only time that you will disclaim the opinion, of course, upon consideration of other things as well.