Choose the sentence that correctly uses the idiom rank and file to refer to ordinary members of a labour union.

Introduction / Context:
This question focuses on the fixed idiom rank and file, which means the ordinary members of an organisation, as opposed to its leaders. The sentence also tests correct preposition use with the phrase have a bond with someone or something.

Given Data / Assumptions:
The intended sense is that the speaker lacks a special connection with the ordinary members of the Labour Union but does know some senior technical people. The incorrect original phrase rank and folder must be corrected to the standard expression. Any change that distorts that idiom or disturbs preposition patterns will be wrong.

Concept / Approach:
The idiom rank and file is a set phrase and should not be modified. It always appears as rank and file, not rank and folders, rank and portfolio, or other variations. The usual collocation is have a bond with or have no bond with, where with introduces the group. Therefore we look for an option that keeps with and inserts the exact idiom rank and file of the Labour Union.

Step-by-Step Solution:
First, read option C. It says rank and file of the Labour Union and keeps the preposition with, giving have any special bond with the rank and file of the Labour Union. This matches both idiom and grammar.Second, read option A. It says rank and folders, which is meaningless in this context and clearly wrong.Third, read option B. It says rank and portfolio, which mixes an idiom with an unrelated word. Portfolio refers to a set of investments or responsibilities, not ordinary members.Fourth, read option D. It changes the pattern to bond in the rank and file with the Labour Union, which is not standard usage and sounds clumsy.Finally, confirm that only option C supplies the correct idiom with correct preposition and natural word order.

Verification / Alternative check:
We can verify our choice by recalling common uses from newspapers and magazines. Writers often say the rank and file of the party, unrest among the rank and file, or support from the rank and file. In every case, rank and file is the exact phrase used to refer to ordinary members. When you mentally substitute the Labour Union for the party in those examples, option C fits perfectly.

Why Other Options Are Wrong:
Option A creates a non existent expression rank and folders, which has no established meaning. Option B uses portfolio, which is a different concept altogether and does not combine with rank to form any idiom. Option D breaks the smooth pattern have a bond with and instead inserts in the rank and file with, which is not how native speakers construct such sentences. All these errors make the alternatives unacceptable.

Common Pitfalls:
Many exam candidates misread fixed expressions and try to guess their components based on spelling or sound, creating forms like rank and folders instead of rank and file. Another pitfall is overcomplicating preposition structures by adding extra words like in, when a simple with would do. To avoid such mistakes, learn idioms as complete units and memorise them in context, and always check whether a phrase you produce actually exists in standard usage.

Final Answer:
The correct sentence is Although I do not have any special bond with the rank and file of the Labour Union, I know some top engineers.