What is the name of the acid which is present in tobacco?
And what will happen if we put calcium hydroxide in tobacco (calcium hydroxide being a base and acid in tobacco)?

I think it's phosphoric acid or one of its derivatives. I don't think you're referring to "nicotinic acid" that is more commonly known as niacin.

If you add a base, it will absorb the acid if there is a mechanism for the reaction to occur (there often is). However, acids make up a significant part of the taste of many substances (not sure about tobacco since I don't smoke). Acids make things taste sour. Remember that the tongue only senses salty, sweet, sour, and bitter. A slight sour taste might be desirable. If you removed the acid, it might make it taste too flat.