1. Mark surfs the Internet every day, but he has never (1)...............more than an hour at a time online.
- Spent is the correct answer here because it's the past participle form of the verb "spend" and is used with "have never" to express a past action that has not been completed.
2. Mark's got a laptop and also a smartphone, so he can (2)....................the Internet anywhere.
- Use is the most suitable option here as it expresses the ability to utilize the Internet.
3. Today, for instance, Mark has been (3)................three times.
- Online is the correct choice as it refers to being connected to the internet.
4. He is not an Internet addict, but some of his friends (4)........................
- Are is the correct present tense form for plural subjects like "friends."
5. One friend of his always looks (5)..............because she spends all night online.
- Tired is the most logical option to describe someone who spends all night online.
6. Although she's got a lot of bad marks on the exams, she hasn't ( 6 )....................her habits.
- Changed is the past participle form of the verb "change" and is used with "hasn't" to express a completed action that hasn't happened.