“fc” is the code for which of the following?

On comparing I and II, we can infer that "is" is the common word where "oi" is the code for it. Similarly, if we compare I and III and I and IV we get the codes for "happy" and "life" as "Im" and "st". 

On comparing IV with II and III, we can see that the common words are "match" and "to". The code which is common between them is "le" and "dk". Hence, the code for "match" is "le" and for "to" it is "dk". The remaining code "rc" is for Bottle and "ky" is for "forever".

The codes for the given words are as follows,

Happy - lm 

is - oi 

match - le 

to - dk 

life - st 

bottle - rc 

forever - ky

 risky/betting = pn/az 

key/be = kr/fc

The code for key/be is kr/fc. fc can be either ‘key” or “be”. Hence, option D is the correct answer.