Receptor is on plasma membrane
a. peptide
b. steroid

a. Peptide hormone.

Explanation:

Hormones can be categorized into two groups based on their binding with the receptors. Some hormones bind to the membrane receptors because they cannot cross the phospholipid membrane. These hormones are peptide or protein hormones. Example is insulin hormone. They bind to exterior or membrane receptors and activate one or more secondary messengers for the physiological change. The other group of hormones are steroid and thyroid hormones which can easily cross the membranes. So they bind to the intra-cellular receptors or they enter nucleus and bind to the DNA for bringing the physiological change.

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