What does 'tied up' mean?

[Svitlana_Morokhovych]

English Language Tests, Intermediate level

ESL/EFL Test #233 "Common Phrases and Responses (1)", question 4

Kristine's working a lot. Are you tied up this week? .........

(a) Yes, I look forward to it.
(b) Yes, I believe you.
(c) Yes, I'm really busy.
(d) Yes, I'm really tired.

English Language Tests, Intermediate level

ESL/EFL Test #233 "Common Phrases and Responses (1)", answer 4

Kristine's working a lot. Are you tied up this week? Yes, I'm really busy.

Correct answer: (c) Yes, I'm really busy.

Your answer was: incorrect
Kristine's working a lot. Are you tied up this week? Yes, I look forward to it.
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I am very sorry, but I really don't understand exactly the meaning of the above statement!
Could someone give me a little bit wider explanation please?

Thank you so much!

[Mister Micawber]

Tied up = busy
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Native English teacher at Mister Micawber's