Six books of various subjects are kept in a shelf one on the top of the other. Electronics book is just above the Chemistry book. Statistics book is between Commerce book and Environmental Science book. The book of Botany is between Electronics and Commerce books. The book of which subject is at the top?

1. Electronics book is just above the Chemistry book. So we have

Electronics
Chemistry 

2. Statistics is between Commerce and Environmental Science. We get two cases for this: 

CASE 1                                                             CASE 2

Commerce                                                     Environmental Science

Statistics                                     OR              Statistics

Environmental Science                                  Commerce

3. The book of Botany is between Electronics and Commerce books. Again, we get two cases for this: Either Electronics books, then Botany and below that, Commerce or Commerce book, then Botany and then below that, Electronics. But the first case is invalid because according to the first statement, the book immediately below the Electronics book is Chemistry, so it can't be Botany. Hence, the arrangement possible is:

Commerce 

Botany

Electronics

Chemistry

Now, if we notice in statement 2, there can't be a statistics book below Commerce, as we know there is a Botany book. Hence, Case 1 is eliminated. The final arrangement is:

Environmental Science

Statistics

Commerce

Botany

Electronics

Chemistry

So, the book at the top is Environmental Science.