If an individual has the genotype dd, what alleles can it pass on to its offspring?

Both "D" and "d" allele

Explanation:

As per the principle of segregation proposed by mendel, an allele pair can split in to two during cell division in a way such that it can  give both of its allele (with in an allele pair) to its offspring.

Thus by that logic anyone with a genotype Dd, can segregate its allele pair to pass both "D" and "d" allele to its offspring.