Which of the following strategies would allow you to determine if a bacterial colony had been infected with a?

I think your question got cut off unless I'm just misunderstanding it. But a,d) there are staining techniques that will allow you to see viruses under electron microscopy and, to a much lesser extent light microscopy (although the light techniques force them to bind together to be big enough to see, not great if you're interested in any kind of diagnostic). b) If viruses are present and you can isolate them on a western blot. c) If the virus is lysogenic, you can run a polyacrylamide gel and the bacterial genome should be larger by some multiple of the length of the viral genome (depending on how many copies were inserted).

It's probably not a lot of help without knowing what kind of virus you're looking for, but hopefully you get something out of it.