We need to check which given boron compounds react with ammonia to form boron nitride, $\mathrm{BN}$.

Recall some standard reactions of ammonia with boron compounds:

• Elemental boron reacts with ammonia on heating: $2\mathrm{B} + 2\mathrm{NH_3} \rightarrow 2\mathrm{BN} + 3\mathrm{H_2}$ So, A is correct.

Check $\mathrm{B_2H_6}$:

Diborane reacts with ammonia first to form an addition compound, which on heating gives borazine / related intermediates, and finally boron nitride is obtained on strong heating. Overall, diborane can give boron nitride with ammonia.

A simplified overall reaction is: $\mathrm{B_2H_6} + 2\mathrm{NH_3} \rightarrow 2\mathrm{BN} + 6\mathrm{H_2}$ Hence, B is correct.

Check $\mathrm{B_2O_3}$:

Boric oxide reacts with ammonia at high temperature to form boron nitride: $\mathrm{B_2O_3} + 2\mathrm{NH_3} \rightarrow 2\mathrm{BN} + 3\mathrm{H_2O}$ So, C is correct.

Check $\mathrm{HBF_4}$:

$\mathrm{HBF_4}$ contains the tetrafluoroborate ion, $\mathrm{BF_4^-}$. In presence of ammonia, it does not give boron nitride directly because the very stable $\mathrm{B-F}$ bonds resist conversion to $\mathrm{BN}$. Instead, acid-base reaction with ammonia would dominate.

Therefore, D is incorrect.

Hence the compounds that react with $\mathrm{NH_3}$ to give $\mathrm{BN}$ are: $\boxed{\mathrm{A,\ B,\ C}}$

Comparison with stored correct answer:

Stored correct answer = A, B, C

Our derived answer = A, B, C

So, the answers agree.