the definite article in front of a noun that is followed by a defining clause [duplicate]

Solution 1:

Why do you think that a noun followed by a deliminiting (defining) clause needs a definite article?

A fox is a mammal that lives in fields and woods.

People who need help in the museum should ask a guide.

A friend [who is] in need is a friend indeed.

The definite article is generally used to identify a specific or unique object.

He is the man who rang the bell.

Of all mammals, it is only the echidna who lays eggs.

In one of your examples, the addition of the changes the meaning of the sentence

Rolo Cookies are the cookies (that are) sold at Tesco?

and

Rolo Cookies are cookies (that are) sold at Tesco?

These are not the same. The first suggests that Rolo may be the only type of cookies sold at Tesco. The second gives no such connotation.

When you are referring to a class or category of things, the definite article is often not used unless that class is unique in the characteristics or actions you are describing.

People who live in glass houses shouldn't throw stones. People who live in brick houses shouldn't either.

but

The people who live in the glass house are the ones who threw the stones.