The principal reason why national economic planning is still being pursued, despite embracing a market economy since 1991, is that Five-Year Plans can continue to provide a long-term perspective to the economy in market-friendly fashions.

After the economic reforms initiated in 1991, India embraced a market-oriented approach to its economy. However, national economic planning has not been completely abandoned. The Five-Year Plans continue to play a role in providing a long-term perspective and direction to the economy. These plans help guide policy formulation, resource allocation, and sectoral development in a market-friendly manner.

While the other options mentioned may have some relevance, they do not capture the principal reason for pursuing national economic planning. Constitutional requirements, the need to nurture capital deployed in the public sector, and the necessity of central planning in agriculture may be factors influencing the continuation of economic planning, but they are not the principal reason.

Therefore, the correct answer is: Five-Year Plans can continue to provide a long-term perspective to the economy in market-friendly fashions.