At the end of a business conference, 10 people present shake hands with each other only once. How many hand shakes will be there all together ?

The correct answer is (b) 45.

To calculate the number of handshakes, we can use the formula $n(n-1)/2$, where $n$ is the number of people. In this case, there are 10 people, so the number of handshakes is $10(10-1)/2 = 45$.

Option (a) is incorrect because it is the number of pairs of people. Option (c) is incorrect because it is the number of ways to choose 2 people from a group of 10. Option (d) is incorrect because it is the number of ways to arrange 10 people in a line.