Arrhenius is a substance that dissociates in water to produce hydrogen ions (H+) and there is a clear answer which coincides with this definition. The correct answer is definitely b. a substance that increases the concentration of hydronium ions when dissolved in water.
Since we're talking about a chain reaction, I think this would be a) self-sustaining. Chain reactions are not usually described as "weak" and can be highly explosive depending upon the compounds (think Uranium!).
The first (artificial) self-sustaining nuclear reaction is attributed to Enrico Fermi in 1942. Here's a bit of history:
http://www.atomicarchive.com/History/firstpile/firstpile_01.shtml
When two waves come in contact with each other while traveling on the same medium, its called constructive interference. Constructive interference happens when two waves come together.
