The question asks which of the following nationalist leaders in India is an exception to writing commentaries on the Bhagavad Gita to argue the case for an ethical foundation to Indian nationalism. The correct answer is option 4, Rammanohar Lohia.

Option 1, Sri Aurobindo, is not an exception because he did write a commentary on the Bhagavad Gita called "The Synthesis of Yoga" which emphasized the importance of spirituality in Indian nationalism.

Option 2, Mahatma Gandhi, also does not fit the exception because he famously wrote his own commentary on the Bhagavad Gita called "Anasakti Yoga" which stressed the significance of selfless action and non-violence in the pursuit of Indian independence.

Option 3, Bal Gangadhar Tilak, is another nationalist leader who wrote a commentary on the Bhagavad Gita called "Gita Rahasya" which promoted the idea of using the text`s teachings to inspire and mobilize the Indian population against British rule.

Therefore, the correct answer is option 4, Rammanohar Lohia, as there is no record of him writing a commentary on the Bhagavad Gita to argue for an ethical foundation to Indian nationalism.