If a homozygous individual expressing the dd genotype for a trait is crossed with a heterozygous dd individual, what percentage of the resulting offspring, on average, would be expected to exhibit the recessive phenotype?

I think you meant the heterozygous individual had the alleles of Dd, not dd, because that would not be heterozygous.

Using the homozygous alleles (dd) and heterozygous alleles (Dd) we can input these into a punnett square.

See punnett square attached. As you can see, when crossing a heterozygous (Dd) parent and homozygous (dd) parent, the offspring have a 100% of exhibiting the recessive phenotype.

All of the offspring have a lowercase d in their alleles, which indicates the recessive phenotype.

the element that provides strength to the exoskeletons of clams and oysters is phosphorus.

this is so because calcium carbonate is the main component in the shells. calcium carbonate is strengthened by phosphorus.

explanation:

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds