The correct answer is (A) Can reproduce freely with each other and form viable seeds.

Explanation:

Reproductive isolation is a key factor in defining a species. If two plants can successfully interbreed and produce viable offspring, they are considered to be members of the same species. This is because a species is defined as a group of organisms that can interbreed and produce fertile offspring. Therefore, option (A) is the correct answer.

Let's look at the other options:

(B) Having more than 90 percent similar genes:

While having similar genes can be an indicator of relatedness, it is not a definitive way to determine if two plants belong to the same species. Different plant species can have a high degree of genetic similarity due to convergent evolution, shared ancestry, or hybridization. Therefore, this option is not a conclusive way to determine species identity.

(C) Looking similar and possessing identical secondary metabolites:

Plants that look similar and possess similar secondary metabolites may belong to the same genus or family, but this is not a definitive way to determine species identity. Many plant species can look similar but belong to different genera, and even different families. Additionally, some plants may have similar secondary metabolites due to convergent evolution, rather than shared ancestry. Therefore, this option is not a conclusive way to determine species identity.

(D) Having some number of chromosomes:

While chromosome number can be a useful tool in determining relationships among plants, it is not a definitive way to determine species identity. Chromosome number can vary among individuals within a species, and even among populations of the same species. Additionally, different plant species can have the same chromosome number, and some plants can even have multiple sets of chromosomes (polyploidy) which can complicate the use of chromosome number in determining species identity. Therefore, this option is not a conclusive way to determine species identity.

In conclusion, the most reliable way to determine if two plants belong to the same species is to observe if they can reproduce freely with each other and form viable seeds.