Boric aicd has polymeric layer structure in which plannar `BO_(3)` units are joined by

Options

A. covalent bonds
B. two centre-two electron bonds
C. coordinate bonds
D. hydrogen bonds.

Explanation

To solve the question regarding the structure of boric acid and how the planar BO3 units are joined, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding Boric Acid Structure**:
- Boric acid is represented by the formula H3BO3. It consists of boron (B) surrounded by three hydroxyl (OH) groups and is often depicted as planar BO3 units.

2. **Identifying the Units**:
- Each BO3 unit can be visualized as a triangular planar structure where boron is at the center and three oxygen atoms are at the corners.

3. **Joining of BO3 Units**:
- The question asks how these planar BO3 units are connected in the polymeric structure of boric acid.

4. **Examining Types of Bonds**:
- We need to analyze the types of bonds that can connect these units. The options given are:
- A) Covalent bond
- B) Two-center two-electron bond
- C) Coordinate bond
- D) Hydrogen bond

5. **Analyzing Hydrogen Bonds**:
- In the case of boric acid, the hydroxyl groups (OH) on the BO3 units can form hydrogen bonds. The oxygen atom of one BO3 unit can interact with the hydrogen atom of another BO3 unit, creating a network of hydrogen bonds.

6. **Conclusion**:
- Therefore, the planar BO3 units in boric acid are joined primarily by hydrogen bonds. Thus, the correct answer is option D: hydrogen bond.

### Final Answer:
The correct answer is **D) hydrogen bond**.