Core concept: Triploidy and sterility

Most cultivated bananas are triploid (3n = 33).

• During meiosis, homologous chromosomes must pair in twos, but with three copies, pairing is irregular.

• This leads to unbalanced gametes that are non‑functional, so fertilization and seed formation fail.

• The banana fruit develops parthenocarpically (without fertilization), giving the familiar seedless fruit.

Thus, their sterility is fundamentally due to triploidy, matching option (3).

Why the other options are incorrect

(1) Male flower-bearing plants are very rare

Banana plants on commercial cultivars typically bear both male and female flowers on the same inflorescence.
Sterility is not due to absence of male flowers but to chromosome imbalance in meiosis.

(2) They lack natural pollinators in the crop plants

Pollination biology can affect seed set, but even with hand pollination, triploid bananas hardly produce viable seeds because of their meiotic problems.
So lack of pollinators is not the primary cause of sterility.

(4) They are a cross of two unrelated species

Many cultivated bananas are indeed hybrids of Musa acuminata (A genome) and M. balbisiana (B genome), but wide hybridization alone does not necessarily cause complete sterility.
The triploid chromosome number of these hybrids is what mainly causes their sterility; related species can still produce fertile diploid or tetraploid progeny.

So, cultivated bananas are considered sterile mainly because they are triploid and therefore seeds are not set, making option (3) the correct choice.