Some bacteria are not easily killed because of

1. Chitinous wall
2. Endospore formation
3. Presence of mesosome
4. High tolerance

Do explain your answer

The Gram-positive bacteria produce endospores during unfavourable conditions like lack of nutrients, high temperature or other climatic stress. The endospores are tough and resistant structures which remain undamaged during stressful conditions. These are dormant structures which allow the bacteria to stop metabolism. They germinate when the conditions are favourable to continue growth. 

Thus, the correct answer is option B