If the turbulence over a period is defined by the sum of each of the differences (in absolute terms) in number of mergers & acquisitions on a year-on-year basis, then which sector is considered most turbulent for the entire period (2001 - 02 to 2006 - 07)?

From the given table, we can calculate two tables. One for mergers and one for acquisitions.
For Mergers,

For Acquisitions,

The turbulence for the Financial Service sector is much higher when compared to the other sectors.

Turbulence for the Financial service sector can be calculated as (201-194)+(201-160)+(160-116)+(193-116)+(193-177)= 185

Similarly, turbulence for the IT and Telecom sector = 108

Turbulence for Food and Beverage = 50

Turbulence for other sectors = 62