This is a time shift sentence, one needs to know that the word ONCE they were considered to "emulate actions without understanding the action’s deeper significance,".
And now they are considered " prompting scientists to question whether other reptiles might also be capable of genuine imitation",

One must pay heed to that shift happen in second part of the sentence however in the first part there was no shift hence we need something similar to clue "without understanding the action’s deeper significance" which is "insensibly" option B.

Since two parts of the sentence are in contrast with each other,the later mentioned about capability of genuine imitation. Hence, the first blank must be "Insensibly" i.e. without understanding action's deeper significance.

Once considered able to only _________ emulate actions without understanding the action’s deeper significance

We need a word for the blank which describes a type of emulation which happens without understanding the action's deeper significance.

The only word available from the choice list is insensibly.

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