Penjelasan:

The activity that is included in GDP is option (d), the services of a mechanic in fixing the radiator on his car.

GDP (Gross Domestic Product) is a measure of the value of all final goods and services produced within a country's borders in a specific period of time, usually a year. It is the sum of consumption, investment, government spending, and net exports (exports minus imports).

Option (a) is not included in GDP because it is a purchase of a used tractor, which is not a newly produced good.

Option (b) is included in GDP as a government spending, but it is not a final good, and it is not clear whether it is produced domestically or imported.

Option (c) is not included in GDP because it is not a production of a good or service, but rather a financial transaction.

Option (d) is included in GDP as a service, which is a final good.

Therefore, the correct answer is option (d).