10) D

Let ‘T’ be the dominant allele and let ‘t’ be the recessive allele.

                              T                                  t

                T          TT                               Tt

                t            Tt                                 tt

We know that recessive allele prevents seed development, which is why all seeds which had ‘tt'(1/4×1000 = 250) are dead. The remaining 750 seeds developed.
Out of these 750 seeds, some have ‘TT’ whilst others have ‘Tt’

From the table, we can see that 2 out of three plants are Tt whilst 1/3 are TT.

Number of heterozygous plants in the 750 mature plants = 2/3 = 66.666..% ≈ 67%