If a coworker needed a pen they might ask you for one, that's why you should keep spares in your leather what?\nOptions:\n- pocket\n- jacket\n- friend's house\n- desk drawer\n- briefcase
Stream of thoughts:
briefcase
Pen is a basic office supply.

People carry basic office supplies in their office bag.

Leather briefcase is the most common office bag.

People keep extra pens in their leather briefcase.

.

So, the final answer is briefcase