The given sentence is in active voice. Its tense is present continuous. The structures for active/passive voices are: 

Active: Subject + is/are/am + verb (ing) + object…

There are some verbs which are followed by a particular preposition and then take an object with them. Such verbs show the state of mind and not any action. Such verbs take appropriate preposition after them instead of by. Some of such verbs are mentioned with their prepositions:

Known to, surprised at, amazed at, startled at, annoyed with somebody, annoyed at something, included in, filled with, tired of etc.

We can consider the following structure for such sentences:

Active: Subject + verb + preposition + object...

While converting such sentences into passive voices, the verb is always followed by the particular preposition. 

Passive: Object + to be + verb (IIIrd form) + preposition + by + subject...

Example: 

Active: The police enquired into the case.

Passive: The case was enquired into.

Active: The children laughed at him.

Passive: He was laughed at by the children. 

The verb in this sentence is a prepositional verb since it carries preposition with it. While changing the voice of the sentence, we do not drop the preposition and it remains unchanged.

Thus, option D is the correct answer.

The given sentence is in active form of simple past tense. The structures for active/passive voices are:

Active: Subject + verb (IInd form) + object...

Passive: Object + was/were + verb (IIIrd form) + by + subject...

The verb (object) in this sentence is a prepositional verb since it carries preposition "to" with it. While changing the voice of the sentence, we do not drop the preposition and it remains unchanged. So, with the help of the above structures, we can convert the given sentence into passive voice as:

My proposal was objected to by them.

The given sentence is in active voice. Its tense is past continuous. The structures for active/passive voices are: 

Active: Subject + was/were + verb (ing) + object…

The given sentence is in active voice. It is simple form of present tense. The structures for active/passive voices are:

Active: Subject + verb (“s“ or “es” with singular noun) + object…

Passive: Object + Is/are/am + verb (IIIrd form) + by + subject…

The given sentence is in active voice. It is simple form of present tense. The structures for active/passive voices are:

Active: Subject + verb (“s“ or “es” with singular noun) + object…

Passive: Object + Is/are/am + verb (IIIrd form) + by + subject…

The imperative sentences which are used to give advice or suggestion should have ‘should + be + V₃’ in the passive voice.

Therefore, the sentence in the passive voice will be option D i.e. ‘The students should be helped to make more informed career choices.’

The given sentence is in past simple tense and its conversion into passive voice is as follows:

Structure of active voice:

Subject + verb (IInd form) + object

Structure of passive voice:

Object + was/were + verb(IIIrd form ) + by + subject

Here losses is plural thus “were” will be used.

Hence, option C is the correct answer.