Step 1: Place Kalpa in the Vedangas.
Kalpa is one of the six Vedangas, the limbs of the Vedas. The six are Shiksha, Kalpa, Vyakarana, Nirukta, Chhandas and Jyotisha. We match each given option with these limbs.
Step 2: Check Phonetics.
Phonetics is the science of sounds and correct pronunciation. That is Shiksha, not Kalpa. So option 1 is wrong.
Step 3: Check Grammar.
Grammar is Vyakarana, whose chief work is Panini's Ashtadhyayi. It is not Kalpa. So option 2 is wrong.
Step 4: Check Social thought.
Kalpa means rule or procedure. Its texts, the Shrauta, Grihya and Dharma Sutras, lay down rituals and the duties of people in family and society. Among the given options, this is closest to social thought. So option 3 is right.
Step 5: Check Etymology.
Etymology is the origin of words. That is Nirukta, with Yaska as the main author. It is not Kalpa. So option 4 is wrong.
Final Answer:
Kalpa, the Vedanga of ritual and social duty, is best described as social thought, which is option 3.
\[ \boxed{\text{Social thought}} \]