In the following statements, one word per statement is underlined. This word can be changed to obtain the correct statement. In which one it should not be changed?

Options

A. In the living world `ul("all")` organisms have DNA as the genetic material which can be transmitted from one generation to another.
B. Purine pyrimidine ratio in Ribose Nucleic Acid can `ul("never")` be equal to 1.
C. The 3rd carbon atom and the 5th carbon atom of `ul("all")`nucleotides in DNA are involved in the formation of phosphodiester bond.
D. Monocyclic nitrogen bases `ul("always")` form weak hydrogen bonds with dicyclic nitrogen bases in B - DNA.

Answer

To solve the question, we need to analyze each statement and determine which underlined word can remain unchanged without altering the correctness of the statement.

### Step-by-Step Solution:

1. **First Statement**: "In the living world, all organisms have **DNA** as genetic materials, which can be transmitted from one generation to another."
- Here, the word "DNA" can be changed to "RNA" because some organisms, like certain viruses, have RNA as their genetic material.

2. **Second Statement**: "Purine-Pyramidine Ratio in **ribose-nucleic acid** can never be equal to one."
- The term "ribose-nucleic acid" can be changed to "deoxyribonucleic acid" (DNA) because the purine-pyrimidine ratio can be equal to one in certain conditions.

3. **Third Statement**: "The third carbon atom and the fifth carbon atom of all **nucleotide** in DNA are involved in the formation of phosphodiester bond."
- The word "nucleotide" can be changed to "nucleic acids" because the statement is not entirely accurate as both the first and third carbons can participate in bond formation.

4. **Fourth Statement**: "A **monocyclic** nitrogen base always forms weak hydrogen bonds with **di-cyclic** nitrogen bases in B-DNA."
- The word "always" in this context is accurate because monocyclic bases (like cytosine and thymine) do indeed form hydrogen bonds with di-cyclic bases (like adenine and guanine) in B-DNA. This statement does not require any changes.

### Conclusion:
The underlined word that should not be changed is "always" in the fourth statement.

### Final Answer:
The correct answer is the fourth statement.