Help the veterinarian complete her observational notes on a very cunning feline by choosing the correct word.

_____, Sir Paws-a-Lot was fast asleep on the radiator, but the slight twitch of his ear whenever the tuna can opened suggested a brilliant deception.

The correct answer is Ostensibly.

"Ostensibly" is a sentence adverb used to indicate that something appears or is claimed to be true, but very possibly is not. Here, it modifies the entire first clause, perfectly setting up the contrast with the cat's true, sneaky wakefulness.

"Consequently" indicates a result, "invariably" means always, and "expressly" means explicitly—none of which fit the context of a convincing but fake nap.