Answer and Explanation:
The correct answer is "d. strengthened."
The word "enthusiasm" refers to a strong and positive feeling towards something. In this context, the sentence suggests that the person's enthusiasm for the subject was enhanced or made stronger. The reason for this is stated as having an inspiring and motivating teacher.
When someone is inspired and motivated by a teacher, their interest and passion for a subject are likely to increase. Therefore, their enthusiasm is strengthened or intensified.
To summarize, the person's enthusiasm for the subject was "strengthened" because of the influence of their inspiring and motivating teacher.