1. Hydrides of group 15 elements are: $\mathrm{NH_3,\\ PH_3,\\ AsH_3,\\ SbH_3,\\ BiH_3}$

2. Statement A: Stability order

   Down the group, the size of the central atom increases and the $E-H$ bond strength decreases. Hence, thermal stability decreases down the group: $\mathrm{NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3}$ So, A is correct.

3. Statement B: Reducing ability

   As we move down the group, the hydrides become less stable and can more easily release hydrogen, so their reducing character increases: $\mathrm{NH_3 < PH_3 < AsH_3 < SbH_3 < BiH_3}$ Thus, B is correct.

4. Statement C: $\mathrm{NH_3}$ strong reducing agent while $\mathrm{BiH_3}$ mild reducing agent

   This is opposite to the actual trend. Since reducing character increases down the group, $\mathrm{NH_3}$ is the least reducing and $\mathrm{BiH_3}$ is the most reducing.

   Therefore, C is incorrect.

5. Statement D: Basicity order increases in the order ...

   For group 15 hydrides, basicity actually decreases down the group because the lone pair becomes less available for donation due to increasing size and lower tendency to bond: $\mathrm{NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3}$ So if the statement says basicity increases down the group, it is incorrect.

6. Therefore, the correct statements are: $\boxed{A\text{ and }B}$

7. Comparing with the given options:

   • Option C = A and B only

Hence, the correct option is: $\boxed{\text{C}}$