t occurs twice in every occurrence of "contest", and half of
1993 is 996.5. So when t occurs at the end of "contest" for the 
996th time, we can conclude the following

a.  there will be 996 complete occurrences of "contest".

b.  there will be 2*996 or 1992 occurrences of t.

c.  there will be 7*996 or 6972 letters.

d.  the 1993rd t will occur in the middle of the next "contest".

e.  the 6972nd letter will be the 996th occurrence of t at the end 
of "contest" and the 1992nd occurrence of t altogether.

Let's start there and go up from there to the 1993rd occurrence 
of t:

The 6972nd letter is the 1992nd occurrence of t.
The 6973rd letter is c.
The 6974th letter is o.
The 6975th letter is n.
The 6976th letter is t, the 1993rd occurrence of t.  

Answer: 6976th, choice D.

Edwin