Question

which item of knowledge did buddha show out of 20 items of knowledge

Answer 1

the four noble truths.

Answer 2

The Four Noble Truths are the 20 items of knowledge. The truths or realities which are understood by the "worthly ones" who have attained Nirvana. The four noble truths contain of the essence of the Buddha's teachings. It was these four principles that the Buddha came to understand during his meditation under the bodhi tree.

Here's the Four Noble Truths are

1. The truth of suffering (Dukkha)

2. The truth of the origin of suffering (Samudaya)

3. The truth of the cessation of suffering (Nirodha)

4. The truth of the path to the cessation of suffering (Magga)

The Buddha is often compared to a physician and then in the first two noble truths he diagnosed the problem and identified its cause. The third noble truth is the realisation that there is a cure. And the last, in the fourth noble truth Buddha set out the Eightfold path, is the prescription, the way to achieve a release from suffering.