Explanation

Seven days are collectively called week.

Explanation

Mankind does not know this.

We use do for plural nouns.

We use does for singular nouns. 

Mankind is a singular noun which means انسان

Explanation

The word family is a plural noun which means خاندان

Explanation

The name of anything is called noun.

Panacea means remedy.

Explanation

Students is a plural noun.

 NOTE: Mastering (A) is a gerund, i.e. a verb form functioning as a noun. But since is already a noun, it is the better choice.

Important is an adjective modifying the noun goal.

Younger is an adjective modifying the noun students.

Explanation

The word Keen is a noun which means eager or anxious.

Antonyms of Keen: Apathetic or lethargic

For example: He was very keen to help.

Explanation

War is a proper noun here.

And is a conjunction.

Were is a verb.

During is a preposition.

Explanation

The word criteria is plural while the word criterion is singular.

We use a singular verb (is) with a singular noun. 

Explanation

Where are my spectacles?

Because Spectacles are plural word.

Explanation