We all found it ------ great fun to talk with paul.

选择B
这句话本是个从句we all found that it is great fun to talk with paul. 句子的主要动词是found,由于be是连系动词,所以将其省略,而that是表明从句地位的连接词,也不承担任何意义的表达,所以也省略.fun是不可数名词,所以不填.
另外也有find sth+adj的用法,如I find him interesting.在这句中,将fun当成形容词,great为副词修饰fun,也能解释通.