The U.N Framework convention on climate change was established ina.18 March 1989b.21 March 1994c.18 March 1991d.18 March 1993

The United Nations Framework Convention on Climate Change (UNFCCC) was established on May 9, 1992, and it was opened for signature in Rio de Janeiro during the Earth Summit. Thus, none of the provided options (a. 18 March 1989, b. 21 March 1994, c. 18 March 1991, d. 18 March 1993) accurately states the date of establishment.

To be more precise:

• The Convention itself was adopted on May 9, 1992.
• It officially entered into force on March 21, 1994.

Therefore, among the options provided, b. 21 March 1994 is the closest and correct option in the context of when the Convention entered into force. However, the establishment date is not represented by any option.