1. The reaction described is the Balz–Schiemann reaction, used to prepare aryl fluorides from aromatic diazonium salts.

2. In this reaction, an aryl diazonium salt is first converted into the diazonium tetrafluoroborate using $HBF_4$: $ArN_2^+X^- + HBF_4 \rightarrow ArN_2^+BF_4^-$

3. On heating, the diazonium tetrafluoroborate decomposes to give the aryl fluoride: $ArN_2^+BF_4^- \xrightarrow{\Delta} ArF + BF_3 + N_2$

4. Therefore, after treatment with $HBF_4$, the required condition is simply heat.

5. Now check options:

   • A: Only heat — Correct
   • B: $NaNO_2/Cu$ — Not used for fluorination here
   • C: $Cu_2O/H_2O$ — Used in other diazonium substitutions, not fluorination
   • D: $NaF/Cu$ — Not the Balz–Schiemann condition

Hence, the correct answer is: $\boxed{A}$