C. It increases and the pH stays constant.
The answer is A. The pH decreases
The cyanide is

A carbon atom has 4 valance electrons and nitrogen has 5. Below is a Lewis-dot-structure of cyanide.
:N≡C.
The carbon atom is still one electron short of having a full octet and so it will seize another electron from almost anything, making the cyanide ion negative and whatever it took the electron from it now positive.
Hi
Alpha decay is a type of radioactive decay.
So, when Alpha decay goes through a process called "radioactive process", two neutrons and two protons comes together and is ejected from the nucleus of a radioactive atom.
Hope this helped :)