To address this question, we need to identify which of the given options is a primary mission under India's National Action Plan on Climate Change (NAPCC). Let us analyze each option:

1. National Solar Mission: This is indeed a primary mission under the NAPCC. Launched in January 2010, the National Solar Mission aims to increase the use of solar energy for power generation and other applications to strengthen India's energy security and contribute to global efforts to combat climate change.
2. National Nutrition Mission: This initiative, also known as Poshan Abhiyan, aims to improve nutritional outcomes for children, pregnant women, and lactating mothers. It is not linked to the NAPCC, as it focuses on health and nutrition rather than climate change.
3. National Rural Livelihood Mission: Also known as Aajeevika, this program aims to reduce poverty by enabling the poor to access gainful self-employment and skilled wage employment opportunities. It is not part of the NAPCC.
4. National Highway Development Project: This project involves the enhancement of the road infrastructure in India and is not related to climate change initiatives under the NAPCC.

Based on this analysis, the correct answer is clearly the National Solar Mission, which is specifically designed to be part of India's efforts under the National Action Plan on Climate Change to harness solar energy and mitigate climate change.