CFR 91.155(a) Class D: 500' below clouds for VFR. SVFR would allow you get closer, if it's available. (They can turn off the beacon under those conditions, but as far as I know they're not required to.)
Thanks for the responses. That clears it up (no pun intended) for me. So, to be safe and legal, at most Class D airports with a 1000' TPA, it would require a ceiliing of at least 1500' and visibility of 3 miles to enter the pattern. Any lower ceiling would require a special VFR clearance,(which may not be allowed if there is any IFR traffic on an approach.)
The best answer is (c). Flying the pattern below TPA risks violating 91.119 minimum altitudes, especially if there's a congested area below the pattern. The "when necessary for takeoff/landing" exception in 91.119 applies only to the necessary climb to and descent from TPA, not to flying level in the rest of the pattern, so (a) would violate 91.155 and (b) would be legal only if there are no "congested areas" below the pattern. And keep in mind that the FAA's definition of a "congested area" is very inclusive, some examples being a highway with "moderate" traffic and a collection of 6 houses in a 1/4-mile area.