Step 1: Mahābhiṣikramana.
Buddha's spiritual journey began when Siddhartha Gautama renounced his lay life for asceticism. This event is called Mahābhiṣikramana and marks the first instance of spiritual quest.
Step 2: Enlightenment of Buddha.
The second most significant event in Buddhism is the attainment of wisdom through intense penance under the Bodhi tree in Bodh Gaya. This is where he became the "Buddha".
Step 3: First sermon in Ṛṣipatana.
After achieving enlightenment, Buddha delivered the first sermon, the Dharmacakrapravartana Sūtra, to five monks in Ṛṣipatana (Sarnath). This signified the commencement of the propagation of the Dharma.
Step 4: First council.
Immediately after Buddha's parinirvāṇa, the first council was convened in Rājagṛha. This council focused on compiling the teachings and monastic discipline.
Step 5: Spread to Śrī Laṅkā.
During the reign of the Mauryan emperor Aśoka, Buddhism was introduced to Sri Lanka by Mahinda and Saṅghamittā. This is considered the final stage of its spread.
Step 6: Conclusion.
The sequence is: (B) Mahābhiṣikramana → (D) Enlightenment → (C) First sermon → (A) First council → (E) Sri Lanka.