The main focus of the text is that more calories are burnt more efficiently whilst walking rather than other forms of activity.
A – Wrong
The age of the people that prefer waking is irrelevant to the focus of the text.
B – Wrong
This statement shifts the focus of the text to one sport: Tennis. We cannot generalize Tennis with other sports. This statement is too specific.
C – Correct
Bear in mind, the question does mention ‘if true’. This means that those who play sports drink beer afterwards (beer has a lot of calories). This provides an explanation as to why people who play sports decrease their BMI by a short margin.
D – Wrong
This statement shifts the focus away from effect of walking on BMI to why people play sport.