Steroid hormones such as estrogen and testosterone are hydrophobic and therefore cannot dissolve in the blood, which is mostly water-based. To reach their target cells and exert their effects, these hormones need to bind to specific receptors. The receptors for steroid hormones are located either in the cytoplasm or the nucleus of target cells.

Cytoplasmic receptors:
- Steroid hormones such as estrogen and testosterone can pass through the cell membrane and bind to cytoplasmic receptors.
- The hormone-receptor complex then translocates into the nucleus, where it binds to specific DNA sequences called hormone response elements (HREs).
- Binding of the hormone-receptor complex to HREs can enhance or suppress the transcription of specific genes, leading to changes in protein synthesis and cellular metabolism.

Membrane ion channels:
- Membrane ion channels are proteins that span the cell membrane and allow the passage of ions such as Na+, K+, and Ca2+.
- Steroid hormones such as estrogen and testosterone cannot bind to membrane ion channels because they are hydrophobic and do not have the charge or shape required for ion channel binding.

Enzyme-linked membrane receptors:
- Enzyme-linked membrane receptors are proteins that span the cell membrane and have an intracellular domain with enzymatic activity.
- Steroid hormones such as estrogen and testosterone cannot bind to enzyme-linked membrane receptors because these receptors typically bind to hydrophilic ligands such as growth factors and cytokines.

G-protein linked membrane receptors:
- G-protein linked membrane receptors are proteins that span the cell membrane and are coupled to intracellular signaling pathways through G proteins.
- Steroid hormones such as estrogen and testosterone cannot bind to G-protein linked membrane receptors because these receptors typically bind to hydrophilic ligands such as neurotransmitters and hormones.

In conclusion, the correct answer is option D - cytoplasmic receptors - because steroid hormones such as estrogen and testosterone bind to receptors located in the cytoplasm or nucleus of target cells.