a) Má okolo dvoch rokov.

I agree with Balenka. "Má okolo dvoch rokov" is not commonly used in Slovak. The reason is that "dva" (two) is very low number to be used with "okolo" (around). You can use "okolo" when you talk about someone older with a greater number of years e.g.: Má okolo 50 rokov or (even better) Má okolo päťdesiatky. "Asi" can be used every time with any number as well as "Tak" which means the same as "asi" in this situation: Má asi dva roky = Má tak dva roky

I would say: Má asi dva roky.