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Humans have primarily five primary tastes that are sour, sweet, bitter, salty, and umami.

Option a is correct because Humans have primarily five primary tastes that are sour, sweet, bitter, salty, and umami. And each taste has only one corresponding type of receptor in other words, each taste has its own receptors which respond to that taste only. Transduction of these five tastes occurs through various mechanisms that show the molecular composition of the tastant. These tastants attach to their respective receptors, thereby exciting the particular specialized neurons associated with them.

For example, a Salty tastant having NaCl gives Na+ that reaches the taste neuron and excite them directly.

Option b is incorrect because Each taste receptors are responsive to only one taste.

Option c is incorrect because Each taste receptors are responsive to only one taste.

Option d is incorrect because Each taste receptors are responsive to only one taste.