B is order 1 if you look at exp 1 and 2. Multiplying B by 4 multiplies the RR by 4.
A is order 2 because if you multiply A by two, the RR is multiplied by 4. For exp 1 and 3, B is multiplied by 8, so dividing the RR and B by 8, you see that dividing A by 2 divides the RR by 4.
For C look at exp 3 and 4. A is constant so can be ignored. B and C are variable. (0.3/0.8)=0.375
You can now multiply B in exp 3 by 0.375 and RR in exp 3 by 0.375, this gives
0.1, 0.30, 0.40, 0.6 x 10-3
From this you can work out the order with respect to C.