Answer is D, ie either one can give the answer.
Look at the rows and columns clearly before reading further.
From S1, v+z=6, so both v=z= 3 ( all are 1, 2 or 3)
only one 3 can be present in any row or column,
eliminate rows and columns containing 3, then we see that only r can take the value of 3,
so, r=3 and s1 is sufficient
From S2, (s+t) + (u+x) = 6, s+t belongs to a row and u+x belongs to a column,
the minimum sum of two numbers in a row/column is 3 (sum 1 +2) (since nos are 1,2 & 3 only)
so, from above given equation we see that s+t=3 and u+x=3
so, the only number in that row/column is r and it should be 3. since we have already taken 1 & 2.
so, r=3 and S2 is sufficient