1. Identify the nature of toilet cleaning liquid

   Most chemical toilet cleaning liquids contain strong acids such as hydrochloric acid. Therefore, if it spills on the hand, the chemical burn is likely due to an acidic substance.

2. Principle of first aid for acid spills on skin

   For an acid spill, we should use a mild base to neutralize it safely. A strong base is avoided because it can itself cause injury.

3. Check the options

   • A: aqueous NaOH
     $\mathrm{NaOH}$ is a strong base. It is corrosive and unsafe for skin first aid.

   • B: aqueous $NaHCO_3$
     $\mathrm{NaHCO_3}$ is a weak base and is commonly used to neutralize acid safely.

   • C: aqueous $NH_3$
     Aqueous ammonia is basic and can irritate/burn skin; not preferred for first aid.

   • D: vinegar
     Vinegar is acetic acid, so it would worsen an acid spill.

4. Conclusion

   The best first aid is aqueous sodium bicarbonate.

$\boxed{\text{B: aqueous } NaHCO_3}$

5. Comparison with stored correct answer

   Stored correct answer = B
   Derived answer = B
   Hence, they agree.