Answer:

Introduction:
In this question, we are asked to choose the correct word to fill in the blank in order to complete the sentence. The sentence states that the person believed someone to be their well-wisher, but it turns out that this person was actually deceiving them. We need to choose the word that accurately conveys the emotions the person may have felt upon discovering this deception.

Explanation:
To determine the correct word, let's analyze the options provided:

A. Must:
The word "must" indicates a strong obligation or necessity. However, in this context, it does not accurately convey the emotions of the person. It implies a logical deduction rather than an emotional response.

B. Ought:
The word "ought" suggests a moral or social obligation. While it can be used to express an expectation, it does not capture the emotional aspect of the situation described in the sentence.

C. Shall:
The word "shall" is used to indicate future actions or to express commands. It does not fit in the sentence contextually and does not convey the appropriate emotion.

D. Would be:
The phrase "would be" is the correct choice in this scenario. It helps convey the emotional impact of the situation. The use of "would be" indicates a hypothetical or imagined outcome, which is appropriate for expressing the person's emotions upon discovering the deception.

Conclusion:
Based on the analysis, the correct answer is option D. "Would be." This choice effectively conveys the emotion of the person upon realizing that their well-wisher was actually deceiving them.