Question

Which choice shows the proper way to use a colon?

Warning: If taken internally, consult a physician immediately.
There is: one thing you should always read a warning label.
Warnings are written: on the following labels medicines, cleaning products, and some foods.
My doctor said, “Read the label.”

Answer 1

Well the second one just seems awkward to stick [ : ] right after "There is" it doesn't make much sense
The third choice the [ : ] is also placed in another awkward spot and doesnt sound right
and the last choice "My doctor said, "Read the label."" doesnt even compete
That leaves you with one choice left.
Warning: If taken internally, consult a physician immediately. <- shows the proper way to use a colon.