Definitly #3, for one the dept may have policy against you carrying, and the officer could get in deep $h!t, as well so could you. Second The officer needs to know for his own benifit. Third, if the SHTF, the officer will probably tell you how to access his shotgun, AR, backup piece, or what ever he has available, some even carry a second handgun for a ride-along should the need arise. I know I did.
As far as carrying in prohibited places, if you are allowed to carry while doing a ride along, you will be allowed to carry in these places since you will be under the direction of the officer in the performance of his duty in most places
Bottom line, talk to the officer and follow his direction in everything that you do, after all he is responsible for your actions during the tour
Stay safe and PAY ATTENTION