Which physical property of di-hydrogen is wrong?

We start by recalling the well-established physical properties of di-hydrogen, the molecular form of hydrogen written simply as $$H_2$$.

First, di-hydrogen is invisible to the human eye, so it is described as a colourless gas. This statement is found in every standard data table of gaseous properties.

Next, di-hydrogen possesses no characteristic smell that can be perceived at ordinary concentrations. Hence it is correctly called an odourless gas.

Similarly, when di-hydrogen is inhaled in small, safe laboratory quantities, it stimulates no taste receptors. Therefore it is properly labelled a tasteless gas.

The crucial property to examine is inflammability. A substance is said to be inflammable (sometimes also termed flammable or combustible) if it catches fire readily in the presence of an ignition source and oxygen.

It is a familiar laboratory demonstration that a mixture of $$H_2$$ and air explodes or burns with a pale-blue flame when ignited:

$$2H_2(g) + O_2(g) \rightarrow 2H_2O(g) + \text{energy}$$

This reaction releases a large amount of heat, showing that di-hydrogen is highly inflammable. In fact, its inflammability is so pronounced that the gas must be handled only in spark-free apparatus and well-ventilated areas.

Therefore, calling di-hydrogen a non-inflammable gas contradicts experimental fact. The statement is incorrect, whereas the other three statements are correct.

Hence, the correct answer is Option D.