Drumroll please...

"Dat." About as close as he can manage to "cat". Sometimes it's just 'at. It's never da', because "da" is a totally different word, used to denote an object or person of interest. Like when he sees a dog, or a truck, or a Christmas tree. Or his mama. Or something he can stick in his mouth. "Da" is always accompanied by a vigorous pointing of the finger. Then again, so is "dat", although "dat" is also followed by a high-pitched squeal and some enthusiastic bouncing. Chins can't really jump yet, so bouncing is about the best he can manage. Still, it's kinda awesome.