Question

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Answer 1

Student drivers are required to keep a log of their supervised driving hours. However, not all students are diligent in their record-keeping.

The transition that should be used to combine these sentences is the following one: however. This adverb is used to introduce a statement that contrasts with or appears to contradict something that has been previously said.  In this case, you are saying that student drivers should keep a written record of their supervised driving hours yet you are also stating that not all of them do it thoroughly and well.

Answer 2

however seems to be the most likely answer 

Student drivers are required to keep a log of their supervised driving hours,however not all students are diligent in their record-keeping.