1. Recall the definition of a Lewis acid

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

So we check each compound for electron deficiency or ability to accept a lone pair.

2. Examine each species

(i) $BCl_3$

• Boron has 3 valence electrons.
• In $BCl_3$, boron forms three covalent bonds and has only 6 electrons around it.
• Hence it is electron-deficient and can accept an electron pair.

Therefore, $BCl_3$ is a Lewis acid.

(ii) $AlCl_3$

• Aluminium has 3 valence electrons.
• In $AlCl_3$, aluminium also forms three bonds and remains electron-deficient.
• It can accept an electron pair.

Therefore, $AlCl_3$ is a Lewis acid.

(iii) $PH_3$

• Phosphorus in $PH_3$ has one lone pair.
• A species with a lone pair tends to donate an electron pair.

Therefore, $PH_3$ is a Lewis base, not a Lewis acid.

(iv) $SiCl_4$

• Silicon in $SiCl_4$ can accept an electron pair from strong donors because silicon has vacant orbitals available in the usual school-level treatment.
• So $SiCl_4$ can behave as a Lewis acid.

3. Evaluate the options

Option A: $BCl_3$ and $AlCl_3$

• Both are Lewis acids.
• So this option is correct.

Option B: $PH_3$ and $BCl_3$

• $PH_3$ is not a Lewis acid.
• So this option is incorrect.

Option C: $AlCl_3$ and $SiCl_4$

• $AlCl_3$ is a Lewis acid.
• $SiCl_4$ can also behave as a Lewis acid.
• So this option is also chemically valid.

Option D: $PH_3$ and $SiCl_4$

• $PH_3$ is not a Lewis acid.
• So this option is incorrect.

4. Comparison with stored answer

From standard Lewis acid-base concepts:

• $BCl_3$ and $AlCl_3$ are definitely Lewis acids.
• $SiCl_4$ also shows Lewis acidic behavior.

Hence, if this is a single-correct MCQ, then the question is ambiguous because Option C is also correct.

So I do not fully agree that only Option A is correct.