What altitude to fly on a STAR when it reads "expect"?
Terrible question (not your question, but the test question).

As far as I can tell, there are NO altitudes on the arrival that are mandatory; everything is an "expect" altitude only.

When you are "cleared for the STELA ONE arrival", you are cleared to (required to) fly the lateral ground track, and comply with any airspeeds stated on the chart ("cross at xxx knots", not "expect to cross at xxx knots" restrictions).  That clearance by itself doesn't clear you for any altitudes, even ones that are mandatory on the chart.

When you are cleared to "Descend Via the STELA ONE" arrival, then you are additionally cleared for all mandatory altitudes.  (Which I don't see any of on this chart, but I may have missed one -- NOS formatting isn't the best.)  Importantly, if the arrival has published altitudes to cross A at or above FL 230, cross B at or below FL 200, cross C at 12,000', and expect to cross D at 5000' (and no other altitudes after D), then you are ONLY cleared down to 12,000' -- you are NOT, in that case, cleared for the 5000' altitude.  You can expect it (i.e. load it in the FMC), but you can't depart 12,000' without explicit ATC clearance (i.e. don't spin  the 5000' into the altitude window on the mode control panel until ATC says "cross D at 5000").

Yeah, it's a "gotcha".  Really poor human factors design for an arrival to do that, IMHO, but there are a few of those out there.

Now, flying the STELA ONE, you're PROBABLY going to end up crossing STELA at 11,000', but until they assign you that altitude, you aren't cleared to do so.  "Expect" altitudes on an arrival have to be explicitly given by ATC -- "Descend Via" doesn't include those.

I don't know what other answers you had on the knowledge test besides 11,000', but my first guess would be that something along the lines of "not enough information given to know" or "cannot be determined" or "last assigned" would be better than "11,000" if the question asked something along the lines of, "having been assigned this arrival, at what altitude are you cleared to cross the STELA waypoint?"  With only clearance for the arrival itself, you haven't been cleared for ANY altitude change.

I don't much like that arrival since it has no mandatory altitudes at all, and I really don't like that question.  Have to love the FAA, sometimes!