What is the reason for the emission of electromagnetic waves from the sun and why curve its in about the visible light is maximum?
If you vibrate a charged object back and forth, it will emit electromagnetic radiation. The atoms and molecules that make up matter vibrate randomly and hence produce radiation. The hotter they are, the more violently they vibrate, and the more energetic (meaning higher frequencies) becomes the radiation they produce. The resulting spectrum of frequencies is called a blackbody spectrum which has a peak in it that we use to define that object's temperature. The surface of the sun emits like a black body and its temperature (~5800K) happens to put the peak of its emission spectrum in the visible range.
"why curve its in about the visible light is maximum?" We could turn this question round and ask why radiation near the peak spectral intensity of the Sun is visible. A Darwinist might reply that eyes have evolved to make use of most intense available radiation.