The correct answer is: C. is
The weather is a condition of the atmosphere, especially in a place and at a particular time, as hot, cold, wet, dry, windy, etc. It is usually described in terms of temperature, humidity, wind speed, and precipitation.
In the sentence “The weather is very pleasant today,” the weather is being described as a current condition. Therefore, the verb “is” is the correct choice.
The other options are incorrect because they are not consistent with the tense of the sentence. The verb “was” is in the past tense, and the verbs “were” and “are” are in the plural form. The weather is not a person or a group of people, so it cannot be plural.
In conclusion, the correct answer to the question “The weather . . . . . . . . very pleasant today” is C. is.