Fill in the blank with the correct form of verb from the options give…

The correct answer is Option (2) → taste

Verbs of the senses as linking verbs:

• Taste, smell, look, sound and feel can mean "have a certain flavour, smell, look ...". Used this way they are linking verbs.
• In this sense they describe a state, so they take the simple present, not the continuous form.
• This sentence:
  • The grapes are described as they are now, so the present tense is needed.
  • The plural subject grapes takes taste, not "tastes".
  • Complete sentence: These grapes taste sour in comparison to what I had yesterday.

Why the Other Options are Incorrect?

Option 1: is tasting — Incorrect.

• "Is" is singular and does not agree with the plural grapes.
• A state verb meaning "have a flavour" is not used in the continuous form. "Is tasting" is used only when a person samples food.
  Example: The chef is tasting the sauce before serving it.

Option 3: tasted — Incorrect. The simple past would describe the grapes at a past time, but "these grapes" are being compared now with the ones eaten "yesterday". The present tense is needed.
Example: The mangoes we bought last week tasted very sweet.

Option 4: have tasted — Incorrect. The present perfect is used for an action completed up to now ("I have tasted Thai food"). It cannot describe the present flavour of the grapes.
Example: I have tasted many kinds of cheese in France.

Contrast: "These grapes taste sour" describes a state, so it takes the simple present. "The chef is tasting the sauce" describes an action, so the continuous form is possible.