What are some of the ways in which diffraction can be useful or troublesome?

i had the same answer and put c. deflect randomly in multiple directions.   i looked it up on google and what i found was this;

scattering of light: by small particles and molecules in the atmosphere. different from reflection, where radiation is deflected in one direction, some particles and molecules found in the atmosphere have the ability to scatter solar radiation in all directions.

i know it's kinda annoying when you are just given a google article but this is why i think it is c.   i will try to let you know if i got it right.