When two chemical species $\mathrm{X}$ and $\mathrm{Y}$ unite, a product $\mathrm{P}$ is formed that contains both $\mathrm{X}$ and $\mathrm{Y}$.
$X+Y\to P$
Simple chemical reactions cannot break down X and Y into simpler compounds. Which of the following statements about species X, Y, and P is true?
(i) P is a compound
(ii) X and Y are compounds
(iii) X and Y are elements
(iv) P has a fixed composition

X and Y can't be split down anymore into simpler elements. As a result, X and Y are elements. P is a compound with a fixed composition since it can be broken down into its constituent parts.

The simplest chemical substances are elements, which are made up of identical atoms.

Compounds are chemical compounds made up of two or more elements linked together chemically in a certain ratio.

Hence, option 4 is correct.