Recall the definition of a Lewis acid

A Lewis acid is a species that can accept an electron pair.

A Lewis base is a species that can donate an electron pair.

Check each option

A: $PH_3$

• Phosphorus has a lone pair in $PH_3$.
• So it tends to donate an electron pair.
• Hence, $PH_3$ is a Lewis base, not a Lewis acid.

B: $B(CH_3)_3$

• In $B(CH_3)_3$, boron has only 6 electrons around it.
• Boron is electron-deficient and has an empty orbital.
• Therefore, it can accept an electron pair.
• So $B(CH_3)_3$ is a Lewis acid.

C: NaH

• Sodium hydride is essentially $Na^+H^-$.
• The hydride ion $H^-$ is strongly electron-rich and donates electron density.
• Hence, it behaves as a Lewis base, not a Lewis acid.

D: $NF_3$

• Nitrogen in $NF_3$ has a lone pair.
• Therefore, $NF_3$ can donate an electron pair.
• So it is a Lewis base, not a Lewis acid.

Conclusion

The only Lewis acid among the given options is: $\boxed{B(CH_3)_3}$

Compare with stored correct answer

Stored correct answer: B

Our derived answer: B

Hence, the answer matches the stored correct answer.