This question might be one of the most frequently-asked questions in basic class.
This is probably because they want to say that they are so. But I wonder why. Haha!
Now, let's check the dictionary. You will see many translations for the word. For example, 怠惰、のらりくらり、尻が重い、怠け者、どら、なまくら、だらける、ぐうたら、どら息子…
So many words. But you need to choose the proper word for the situation. And these are factors to consider:
1) Spoken or written word
2) New or old word
3) Feminine or male speech
4) Grammar ( Noun, adjective, verb, or adverb )
When you choose the proper word, you need to take into account all of the above. Difficult ? Yes, I think so too, especially in basic level when students have not acquired the skill on how to consider the right word to use. But to speak natural Japanese, I hope you will be able to take them into consideration.
The answer is ‘わたしは怠（なま）け者（もの）です.’
For instance, ‘怠惰（たいだ)’ is used in formal situation, But you want to say "I’m lazy" in casual situation.
'尻（しり）が重（おも）い' means ‘be slow to act’
‘どら’、’なまくら’、’どら息子’ may sound a bit too old and funny.
‘のらりくらり（する）’、'なまける'、'だらける' is used to describe situation when ‘you are idling’ . Sometimes, even hardworking people idle on Sunday, don't they? Hahaha.
So that's way ‘ わたしは怠（なま）け者（もの）です ’ is the best answer.
☆ Advanced level Question How do you say "I'm lazy to do 〜". Answer 〜するのが面倒（めんどう）だ。 Example 宿題をするのが面倒だ。