By the reign of which ruler had Gautama Buddha been elevated from a teacher to the position of a worshipped God in Buddhist tradition?

Introduction / Context:
This question explores the development of Buddhist religious thought from the time of the historical Gautama Buddha as a teacher to the later phase when he came to be worshipped as a divine figure. The transformation is closely associated with the rise of Mahayana Buddhism. Identifying the ruler under whose reign this major doctrinal shift gained strong support is important for understanding how religion and royal patronage interacted in ancient India.

Given Data / Assumptions:

• The focus is on the elevation of Gautama Buddha to the position of God, rather than just a spiritual teacher.
• The options list Harsha, Kanishka, Chandragupta Vikramaditya, and Ashoka.
• We assume the question refers to the phase when Mahayana Buddhism expanded with strong royal backing.

Concept / Approach:
Mahayana Buddhism, which emphasised the worship of Buddha as a divine being and introduced the concept of Bodhisattvas, grew significantly under the Kushana ruler Kanishka. Ashoka was an earlier Mauryan emperor who patronised Buddhism and spread it widely but still within a largely earlier doctrinal framework. Harsha and Chandragupta Vikramaditya belonged to later periods where different religious trends existed. Therefore, we should link the theological elevation of Buddha to the reign of Kanishka.

Step-by-Step Solution:
Step 1: Recall that Ashoka supported Buddhism in the 3rd century BCE and helped spread it across Asia but did not belong to the Mahayana phase. Step 2: Recognise that Kanishka, a Kushana ruler, reigned in the early centuries CE, when Mahayana doctrines were flourishing. Step 3: Understand that Mahayana Buddhism presented Buddha as a divine saviour and encouraged image worship and devotional practices. Step 4: Note that major Buddhist councils and doctrinal developments under Kanishka contributed to this elevation of Buddha to Godlike status. Step 5: Harsha and Chandragupta Vikramaditya had different religious priorities and came after these developments, so they are not the correct answer.

Verification / Alternative check:
Textbooks on Indian religions and history typically associate the spread and institutionalisation of Mahayana Buddhism with Kanishka. Checking the timeline of rulers against the evolution of Buddhist schools confirms that he is the most appropriate choice.

Why Other Options Are Wrong:
Ashoka was a major Buddhist patron but lived too early for the fully developed Mahayana phase, so Buddha was still mainly honoured as an enlightened teacher. Harsha was an emperor of the 7th century CE, much later and known for eclectic patronage. Chandragupta Vikramaditya was a Gupta ruler, chiefly associated with a Hindu revival. None of these match the doctrinal turning point in question.

Common Pitfalls:
Many learners pick Ashoka because his name is strongly linked with Buddhism, ignoring the chronological difference between Theravada or earlier schools and Mahayana. To avoid this error, always relate doctrinal changes to their proper historical period and rulers.

Final Answer:
Gautama Buddha was elevated to the position of a God by the time of Kanishka.