The correct option is D The mobile phones we use nowadays are lighter than the ones we used before.
An adjective is usually placed before the noun that it modifies. This is true for adjectives that are in the attributive position, i.e., it comes before the noun (red leaves). When adjectives are in the predicative position, i.e., it comes after the noun, the adjective does not follow it immediately. The adjective is connected to a noun via a linking verb (The leaves are red.).
In the given sentence, the underlined word 'lighter' is an adjective, it describes the weight of mobile phones but the phrase "are than the ones" is missing a word. The question that arises is 'are what?' ('are' is a linking verb). Hence the given sentence needs to be changed and option A is incorrect
When 'lighter' is placed after 'are', then the sentence makes complete sense. 'Lighter' is an adjective in the predicative; it is used for comparison between 'mobile phones nowadays' and 'ones used before' and option B is correct. ('Ones' is a pronoun that refers to 'mobile phones'.)
Option C is incorrect because the adjective is placed before the linking verb. (lighter are) An adjective that is predicative, does not come before the linking verb "are", it comes after it.
Option D is incorrect because the adjective "lighter" must be placed immediately after the linking verb (when the adjective is predicative). It does not modify the verb "used", hence it will not be placed before the verb.
Note: 'Are' is a linking verb, it does not function as typical verbs do. Linking verbs connect the subject with an adjective or its equivalent, that describes the subject.