The guards were ___ at their posts.

The correct choice is 'stationary,' which describes the guards as being motionless or not moving at their posts. The context implies that the guards are standing still, which aligns perfectly with the meaning of the word 'stationary.' This version of the word is used to convey the idea of being fixed in place, particularly in a setting where one would expect guards to remain vigilant and in position, without engaging in any movement.

The other options do not provide the correct meaning in this context. The misspelling 'stationnery' is not a valid English word, while 'stationnary' is also an incorrect spelling of 'stationary.' Lastly, 'stationery' refers to writing materials, such as paper and envelopes, which does not fit the intended meaning related to the guards. The usage of 'stationary' in this scenario is crucial to conveying that the guards are standing still, fulfilling their duty effectively.