To solve this problem, we need to follow the operator @ definition which is a@b = (ab)^(1/2).

Step 1: First, we calculate the operation inside the parentheses, which is 16@81. According to the definition, this equals to (16*81)^(1/2).

Step 2: Multiply 16 and 81 to get 1296.

Step 3: Take the square root of 1296, which is 36.

Step 4: Now, we substitute 36 back into the original equation in place of 16@81, so we have 36@49.

Step 5: According to the definition of @, this equals to (36*49)^(1/2).

Step 6: Multiply 36 and 49 to get 1764.

Step 7: Take the square root of 1764, which is 42.

So, (16@81)@49 = 42. Therefore, the answer is (B) 42.