Answer:
d. an obligate opportunistic pathogen.
Explanation:
Mycobacterium tuberculosis is a rod shaped bacterium which causes tuberculosis. Most of the pathogenic bacteria are obligatory pathogen. It is airborne bacterium which is efficiently spread from person to person through droplet nuclei reach the terminal air-spaces. From air space the mycobacterium ingested by alveolar macrophages and then carried by lymphatics to regional lymph nodes. It is very small and does not get caught in the mucociliary ladder and it is enough large to be immediately exhaled, so, they lands in the alveoli. Thus, it is an opportunistic obligate pathogen.