What is an example of a microorganism acting in a harmful way? a) bacteria aiding digestion. b) bacteria causing strep throat. c) bacteria used to create yogurt. d) fungi decomposing waste for compost

Not very often, but sometimes, the language gives away the question. This is one such instance.

Look at A. Is digestion a bad thing? It shouldn't be. Otherwise you run marathons with gobs of food not being broken down and providing energy. 
Not A.

Not C. Do you like yogurt? If you don't do you know someone who does? Is yogurt for you/them a bad thing. Bacteria here are good. Again, not C.

Not D. Waste is bad. It's a problem. Getting rid of it is good. Again not D.

There's only one left.
B <<<< answer. Who would like to get strep throat. It's a fate for your worst enemy.

B <<<< answer.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).