Identity equation

$\forall x \in \mathbb {N}: 9^x \in \mathbb {Z} \text { and } 6.3^x = \frac {63^x}{10^x} \notin \mathbb {Z}$

(since 63 and 10 are coprimes and so are all their (positive integer) powers)

Therefore, we can close out 5 and 6.

For x = 0, we get:

$9^0 - 6.3^0 - 55 = 1 - 1 - 55 = - 55 ≠ 0$

Hence, our answer is:

$\boxed { \text {None of these.} }$