Answer:

The offspring of mustard plants in field A produced brown coloured seed whereas the offspring of mustard plants in field B produced variation in colours. The probable reason for this will be that the brown colour phenotype is a dominant character and the yellow colour phenotype is a recessive character. The plants in field A will have dominant genotype (BB) and plants in field B will have recessive genotype (bb). Therefore, the cross-pollination between the plants will produce only dominant character in field A and both dominant and recessive character in field B.

Explanation:

The plants of field A with genotype (BB) and brown seeds when cross-pollinated with plants of field B with genotype (bb) and yellow seeds resulted in plants with genotype (Bb), hence the progeny were all brown. Also, the self-pollination in this case (BB x BB) resulted in brown seeds only.

Whereas,  plants of field B with genotype (bb) and yellow seeds when cross-pollinated with plants of field A with genotype (BB) and brown seeds resulted in plants with genotype (Bb), hence the progeny were all brown. Also, the self-pollination in this case (bb x bb) resulted in yellow seeds. Therefore, the progeny in field B were of varied colours.

Answer:

the offspring of mustard plant in field A produced brown coloured seed wheare as in field B it produce variations in colour