The correct answer is (a). The State shall endeavour to protect and improve the environment.
Directive principles of state policy are fundamental principles of state policy that are enshrined in the Constitution of India. They are not enforceable by any court, but the state is supposed to follow them in making laws and policies.
The directive principle of state policy to protect and improve the environment is contained in Article 48A of the Constitution. It states that the state shall endeavour to protect and improve the environment and to safeguard the forests and wildlife of the country.
The other options are not directive principles of state policy.
(b) The State shall not deny to any person equality before law. This is a fundamental right guaranteed by Article 14 of the Constitution.
(c) The State shall not discriminate against any person on grounds of religion, race, caste, sex or place or birth. This is also a fundamental right guaranteed by Article 15 of the Constitution.
(d) Untouchability enforcement is a practice that is prohibited by the Constitution. It is not a directive principle of state policy.