早く起きる___だったのに、寝坊してしまった。
Q. Which word completes 'I intended to get up early (but...)'?
Translation
I meant to get up early, but I overslept.
Explanation
つもりだった expresses a past intention that failed. はずだった would frame it as an expectation about circumstances rather than the speaker's own plan — possible, but the personal-intention reading with 寝坊 makes つもり the best fit.