The Indian Ocean Naval Symposium (IONS) is indeed a voluntary initiative that aims to enhance maritime cooperation among the navies of the littoral states of the Indian Ocean Region, making statement 2 in the question correct. This rationale contributes to option 2, `2 only`, being the accurate answer.

However, the inaugural IONS was not held in India in 2015 under the chairmanship of the Indian Navy, as suggested in statement 1. In fact, it took place in India in 2008, so the first statement is incorrect.

Summarizing, option 1 "1 only" is incorrect because the inaugural IONS wasn`t held in 2015. Option 3 “Both 1 and 2” is also incorrect, due to the inaccuracy of statement 1. Lastly, option 4 “Neither 1 nor 2” isn`t accurate because statement 2 is indeed correct.