Who among the following is the earliest available Indian dramatist?

The correct option is Bhasa.

To understand why Bhasa is considered the earliest available Indian dramatist, we can look at the historical timeline of ancient Indian playwrights:
1. Bhasa (circa 3rd to 4th century BCE or early CE): Bhasa is widely recognized as one of the earliest known Sanskrit dramatists. His existence predates Kalidasa. In fact, Kalidasa himself pays tribute to Bhasa in the introduction of his first play, Malavikagnimitram, referencing Bhasa as a famous predecessor of great reputation. The discovery of thirteen of his plays (such as Svapnavasavadatta) by T. Ganapati Sastri in 1912 confirmed his pivotal place in Sanskrit literature.
2. Kalidasa (circa 4th to 5th century CE): Often considered the greatest poet and dramatist of Sanskrit literature, Kalidasa lived during the Gupta Empire, which is chronologically much later than the period attributed to Bhasa.
3. Bhavabhuti (circa 8th century CE): A major dramatist of the later Sanskrit classical period, Bhavabhuti wrote famous plays like Malatimadhava and Uttararamacharita during the early medieval era.
4. Rajashekhar (circa 9th to 10th century CE): An eminent poet and dramatist who served as the court poet of the Gurjara-Pratiharas, Rajashekhar belongs to a much later period than all of the above.

By comparing these historical timelines, we can clearly see that Bhasa is the earliest available Indian dramatist among the choices provided.