Resident of State X

Property (rental on E, active, RE professional) is in State Y (Nonresident.)

Home office deduction is allowed to be sourced to X or Y?

I suppose the states might matter so I'll list them:  X = CA, Y = RI.

Based on this passage from the RI Nonresident instructions, the answer is State Y because the home office is "connected" to RI.

INCOME OF A NONRESIDENT SUBJECT TO TAX
A nonresident is subject to tax on all items included in his or her total federal income (including his or her distributive share of partnership income or gain and his or her share of estate or trust income or gain) which are derived from or connected with Rhode Island sources as follows:
•From real or tangible personal property located in the state.

(other items omitted by me.)