The meaning of the phrase "conscientious hour" in the below paragraph could be made more explicit if the phrase were restated as:

Paragraph: I took dinner usually at the Yale Club--for some reason it was the gloomiest event of my day--and then I went upstairs to the library and studied investments and securities for a conscientious hour. There were generally a few rioters around but they never came into the library so it was a good place to work. After that, if the night was mellow I strolled down Madison Avenue past the old Murray Hill Hotel and over Thirty-third Street to the Pennsylvania Station.

The meaning of the phrase "conscientious hour" in the above paragraph could be made more explicit if the phrase were restated as:

a. conscientiously spent hour

b. careful hour

c. responsible hour

d. conscientiously timed hour

e. honest hour