【正确答案】 C

【答案解析】这句话的意思是：这位教授全身心投入实验，以至于几乎没时间与家人去度假。以so这一副词开头的句子，句子的主谓要部分倒装。这个句子的正常语序为The professor has become so involved in his experiments that he can hardly have time to go on holiday with his family，所以答案为C。