Griffith called the process he observed transformation because

Griffith called the process transformation because the harmless bacteria had been transformed. So the answer is 'C'.

Explanation:

Griffith performed an experiment on the mouse in which he worked on two different strains of streptococcus. One of the strain  was capable of causing disease (virulent) III-S and the other was a rough strain which was non virulent type II-R. He injected different combinations of the strains. The heat killed smooth strain became non virulent.

The most important discovery he made was that when heat killed smooth strain and rough strain were introduced together in the mice and the mice died. It happened because the rough strain of bacteria transformed into virulent form as the DNA from heat killed smooth strain was imbibed by the rough strain which converted into virulent form.