Final answer:
The word 'disappointment' best fills the blank in the sentence about losing an important election, as it captures the sentiment of losing something of high importance.
Step-by-step explanation:
You can well imagine my disappointment at losing such an important election by so few votes. The appropriate word to fill in the blank is 'disappointment,' as it conveys the sense of sadness and let-down one might feel after not succeeding in something that held significant value, such as an important election. 'Dismay' might also fit, but it implies a stronger, more sudden and alarming sense of defeat, while 'chagrin' suggests a feeling of vexation marked by disappointment or humiliation, and 'frustration' indicates a feeling of being upset or annoyed as a result of being unable to change or achieve something. Given the context of an election, where the stakes are high and the outcome is critically important, 'disappointment' is the most fitting choice.