Which one of the following is a triploid cell ?

Synergids are haploid ($n$) cells present in the embryo sac.

The central cell contains two polar nuclei ($n + n$) which fuse to form a diploid secondary nucleus ($2n$).

The zygote is a diploid ($2n$) cell formed by the fusion of a male gamete ($n$) with the egg cell ($n$).

The primary endosperm cell is formed by the fusion of a male gamete ($n$) with the two polar nuclei or secondary nucleus ($2n$) of the central cell. This process is called triple fusion, resulting in a triploid ($3n$) primary endosperm cell.

Answer: Primary endosperm cell