Answer:

Introduction:
The given sentence is asking us to choose the correct word to complete the sentence, which expresses the idea of serving our nation. Let's analyze the options and understand why option 'A' is the correct answer.

Analysis of Options:

a) ought: This word implies a moral obligation or duty. It suggests that serving our nation is something we ought to do, based on our sense of what is right.

b) must: This word indicates a strong necessity or requirement. It implies that serving our nation is something that is mandatory or obligatory.

c) can: This word emphasizes the ability or possibility to serve our nation. It suggests that we have the capability to serve our nation if we choose to do so.

d) none of these: This option indicates that none of the given words are suitable to complete the sentence.

Explanation of Correct Answer:

The correct answer to the given question is option 'A' - ought. This is because the word "ought" expresses a sense of moral duty or obligation. By choosing this word, the sentence conveys the idea that serving our nation is something we should do because it is the right thing to do. It implies that serving our nation is not just a choice or a possibility, but a responsibility that we have towards our country.

Conclusion:

In conclusion, the word "ought" is the correct choice to complete the given sentence, as it best expresses the notion of serving our nation as a moral duty or obligation. The other options do not convey the same sense of responsibility and duty.