Power supply for an irrigation system
#3 is right out.  The solenoids are designed for and expect 24VAC.  Powering them from DC will probably burn the windings open in short order.

I'd  go with #2, and as far as triggering the triacs go, use an optocoupler designed for triggerring triacs.  There are many out there, you can even get them in zero-crossing or non-zero crossing varieties.  Depending on the load, you might get away with just an opto-coupler triac, without it driving a larger one, but I don't know what kind of load your solenoids are.  Vishay makes the VO2223, which is rated for up to 900mA on the load side.