You must be more motivated.

[keannu]

a is the answer for this question, but my student asked me if d could be an answer, but I said because of comparison of "more motivated", it can't be an answer. Maybe "You must motivated" can be an answer, but I'm not sure, What do you think?

M:I'm studying for tomorrow's exam. I can't slack off
W:________a________
a.Keep your nose to the grindstone
b.Provided it's part of the lecture notes.
c.Ask a classmate to explain the material
d.You must be more motivated.

[dut_thinker]

a is the answer for this question, but my student asked me if d could be an answer, but I said because of comparison of "more motivated", it can't be an answer. Maybe "You must motivated" can be an answer, but I'm not sure, What do you think?

M:I'm studying for tomorrow's exam. I can't slack off
W:________a________
a.Keep your nose to the grindstone
b.Provided it's part of the lecture notes.
c.Ask a classmate to explain the material
d.You must be more motivated.

"Keep your nose to the grindstone" is an informal expression. It means "to work in earnest". So I don't think d can be an answer.

'a ' isthe answer to this question.
You like 'but' so much.

not a native speaker; not a teacher;

[keannu]

"Keep your nose to the grindstone" is an informal expression. It means "to work in earnest". So I don't think d can be an answer.

'a ' isthe answer to this question.
You like 'but' so much.

not a native speaker; not a teacher;

There must be another reason why d can't be an answer.

[Barb_D]

Clearly the student IS motivated. He has just said he's studying and can't slack off.

A: I'm in the midst of a 5-mile walk.
B1: Youl should get more exercise. -- Does that make sense?
B2: Keep on going! -- That makes sense.

In your examples, D would be like B1. Why tell someone they need more motivation when they are right in the midst of doing what needs to be done? A is like B2 - a word of encouragement to keep at it.

[keannu]

Clearly the student IS motivated. He has just said he's studying and can't slack off.

A: I'm in the midst of a 5-mile walk.
B1: Youl should get more exercise. -- Does that make sense?
B2: Keep on going! -- That makes sense.

In your examples, D would be like B1. Why tell someone they need more motivation when they are right in the midst of doing what needs to be done? A is like B2 - a word of encouragement to keep at it.

Does this "must" mean "obligation(have to)" or "certainty" in this context?

M:I'm studying for tomorrow's exam. I can't slack off
d.You must be more motivated.

[Barb_D]

Obligation. LIke the "should in my example.

If it was a certainty, it the "more" wouldn't make sense.

You must be a very motivated student! -- THAT would make sense, because I'm commenting that based on my observation of you, it's the only logical conclusion. You're studying hard, you're obviously self motivated.