Option B: 'Used to' refers to a past habit habit that is no longer true.
It agrees to the given sentence, which mentions a past habit 'sometimes force himself to work...'
This modal completes the sentence in meaning, hence option B is correct.
Option A: 'Could' expresses possibility.
Options C and E: 'Ought to' and 'should' express obligation or duty.
Option D: In 'would be', 'would' expresses a past habit, but 'be' refers to a continuous action. The action mentioned here isn't continuous.
These options are not relevant to the sentence in meaning.
Hence options A, C, D and E are wrong.