If a diploid female plant and a tetraploid male plant are crossed, the ploidy of the endosperm shall be __________.

The correct option is B tetraploid
Diploid female plant will have 2 polar nuclei (each haploid) with which one male gamete from tetraploid male plant (male gamete of tetraploid plant will be diploid) fuses. It forms the Primary Endosperm Nucleus (PEN) which is tetraploid. The PEN further results in the formation of the endosperm and it is tetraploid.
Male gamete (2n) + 2 polar nuclei (n+n) → Primary Endosperm Nucleus(4n).