To determine the order of hereditary similarity from increasing to decreasing, we need to understand the genetic relationships among the given options: (a) Cousins, (b) Identical twins, (c) Siblings, and (d) Fraternal twins.

• Identical twins (b): Identical twins are formed when a single fertilized egg splits into two embryos. They share 100% of their DNA, making them genetically identical. Therefore, they have the highest hereditary similarity.
• Fraternal twins (d): Fraternal twins are formed from two different eggs fertilized by two different sperms. They share about 50% of their DNA, similar to any other siblings, but they are conventionally considered closer in genetic similarity immediately after identical twins.
• Siblings (c): Siblings share roughly 50% of their DNA, resulting from the combination of half the genes from each parent. They have a genetic similarity equivalent to fraternal twins.
• Cousins (a): Cousins share about 12.5% of their DNA, as they share one set of grandparents. This degree of relatedness is the lowest among the given relationships.

Based on this information, the order of hereditary similarity from increasing to decreasing is:

1. (a) Cousins
2. (c) Siblings / (d) Fraternal twins
3. (d) Fraternal twins / (c) Siblings
4. (b) Identical twins

Given the options, the correct choice is: (b), (d), (c), (a). This option correctly reflects the order of genetic similarity from the most similar to the least similar.