The functions of urban green space to slow down the heat island effect are as follows: a. regulating climate; b. reducing noise; c. conserving water and soil; d. conserving water resources.

Problem analysis: this topic examines the impact of biology on the geographical environment. The functions of green plants are: wind and sand fixation, soil and water conservation, water conservation, smoke and dust elimination, noise elimination, air purification and so on. The main function of urban green space to alleviate the heat island effect is to adjust the climate. So choose option a in this question.