The poetry that is sung and appreciated by Dalits, marginalised groups, and those critical of social hierarchies in U.P., Rajasthan, Punjab, Madhya Pradesh, Bengal, Bihar, and Gujarat is Kabir's poetry.
Reasoning:
Kabir, a 15th-century Indian mystic poet and saint, is known for his thought-provoking and socially conscious poetry. His verses challenge social norms, caste-based discrimination, and religious dogmas, making them resonate with the marginalized and oppressed communities. Kabir's poetry emphasizes the unity of humanity and promotes a message of love, equality, and social justice. His works have been widely sung and appreciated by various communities across different states in India, especially by Dalits and those critical of social hierarchies.
Therefore, the correct answer is option C: Kabir's.

Introduction:
Among the notable poets whose poetry is sung and appreciated by Dalits, marginalised groups, and those critical of social hierarchies in various states of India are Tulsidas, Surdas, Kabir, and Rahim. However, the correct answer to the given question is Kabir.

Explanation:
Kabir was a 15th-century Indian mystic poet and saint who is revered by people from diverse backgrounds. His poetry resonates with the struggles and aspirations of the marginalized sections of society, making him an influential figure in various states.

Poetry sung and appreciated by Dalits and marginalised groups:
1. Uttar Pradesh (U.P.): Kabir's poetry is widely sung and appreciated by Dalits and marginalised communities in Uttar Pradesh. His verses, which emphasize equality, social justice, and unity, have been instrumental in uniting people across caste and class barriers.

2. Rajasthan: In Rajasthan, Kabir's poetry has gained popularity among Dalits and other marginalized groups. His writings, which challenge social hierarchies and advocate for the upliftment of the oppressed, resonate with the struggles faced by these communities.

3. Punjab: Kabir's poetry has found a significant following among Dalits and marginalised groups in Punjab. His verses, often sung in the form of folk songs, highlight the importance of inner spirituality and denounce social discrimination.

4. Madhya Pradesh: Kabir's poetry is cherished by Dalits and marginalised communities in Madhya Pradesh. His verses, which critique social inequalities and advocate for the unity of all humankind, have become an integral part of the cultural fabric of the state.

5. Bengal: Even in Bengal, Kabir's poetry is celebrated by Dalits and marginalised groups. His writings, known for their simplicity and universal appeal, have given voice to the aspirations and struggles of the oppressed communities in the state.

6. Bihar: Dalits and marginalised groups in Bihar have embraced Kabir's poetry. His verses, often sung as part of religious gatherings and cultural events, have served as a medium of expression for the oppressed communities in the state.

7. Gujarat: Kabir's poetry has found resonance among Dalits and marginalised groups in Gujarat. His writings, which challenge social norms and advocate for inclusivity, have been instrumental in fostering a sense of unity among these communities.

Conclusion:
Kabir's poetry holds immense significance for Dalits, marginalised groups, and those critical of social hierarchies in various states such as U.P., Rajasthan, Punjab, Madhya Pradesh, Bengal, Bihar, and Gujarat. His verses continue to inspire and uplift these communities, promoting the values of equality, justice, and social harmony.