Another publisher bet Geisel that he could not write an entire book using only 50 different words. Geisel won the bet by publishing the classic Green Eggs and Ham.

According to the passage, which of the following is true about Theodore Geisel?
1) The Cat in the Hat was his first children’s book.
2) He illustrated all of his own books.
3) He wrote Green Eggs and Ham using only 50 words or less.
4) He was not a successful author until the publication of The Cat in the Hat.
5) He only wrote books with limited vocabularies.

(C) CORRECT. Green Eggs and Ham was the book that Geisel wrote in order to win the bet that challenged him to write a book using only 50 words.

"Using only 50 different words" is not same as using only 50 words.
One can use only 50 different words but have more than 50 words.
I choose (4) that is also not very clear in meaning just as (3)

That was my mistake. It is a pessage from CAT test and following is a sentence from that:

Another publisher bet Geisel that he could not write an entire book using only 50 different words. Geisel won the bet by publishing the classic Green Eggs and Ham.

According to the passage, which of the following is true about Theodore Geisel?
1) The Cat in the Hat was his first children’s book.
2) He illustrated all of his own books.
3) He wrote Green Eggs and Ham using only 50 words or less.
4) He was not a successful author until the publication of The Cat in the Hat.
5) He only wrote books with limited vocabularies.

(C) CORRECT. Green Eggs and Ham was the book that Geisel wrote in order to win the bet that challenged him to write a book using only 50 words.

"Using only 50 different words" is not same as using only 50 words.
One can use only 50 different words but have more than 50 words.
I choose (4) that is also not very clear in meaning just as (3)

You are absolutely right! We'll edit the question - I'll alert our curriculum director. Thanks for the catch!