Which of the following pairs of chemical species consists entirely...

Lewis acids are chemical species that can accept an electron pair.

$SiF_4$ has empty d-orbitals in the valence shell of the central silicon atom, allowing it to accept electron pairs and expand its octet (for example, forming $[SiF_6]^{2-}$). Thus, it acts as a Lewis acid.

$BF_3$ is an electron-deficient molecule with an incomplete octet on the central boron atom, making it a strong electron pair acceptor. Thus, it also acts as a Lewis acid.

In the other options:
$H_2O$ is a Lewis base due to the lone pairs on the oxygen atom.
$NH_3$ and $CN^-$ are Lewis bases as they can donate electron pairs.
$Cl^-$ is a Lewis base as it can donate an electron pair.

Therefore, the pair consisting entirely of Lewis acids is $SiF_4$ and $BF_3$.

Answer: $SiF_4$, $BF_3$