`"Colemanite" + Na_(2)CO_(3) overset("Fused")rarr (A) + (B) + CO_(2)`
`underset("Solution")((A)) + CO_(2) rarr(B) + Na_(2) CO_(3)`
`(B) + conc HCl rarr NaCl + underset("Acid")((C))`
`underset("Acid")((C)) +H_(2)O rarr underset("Acid")((D))`
`(D) overset("Strong heating")rarr (E)`
`(E) +CuSO_(4) overset("Heated") underset("inflame")rarr underset("Blue-coloured compound")((F))`.
Compound `(B)` is.

Options

A. `Na_(2)B_(4)O_(7)`
B. `NaBO_(2)`
C. `Na_(3)BO_(3)`
D. `NaOH`

Explanation

To solve the question step by step, we will analyze the reactions provided and identify the compound (B) in the process.

### Step-by-Step Solution:

1. **Identify the Initial Reactants**:
The first reaction involves "Colemanite" (Ca2B6O11) reacting with sodium carbonate (Na2CO3) when fused.

2. **Determine Products of the First Reaction**:
The reaction can be written as:
\[ \text{Ca}_2\text{B}_6\text{O}_{11} + \text{Na}_2\text{CO}_3 \xrightarrow{\text{Fused}} (A) + (B) + \text{CO}_2 \]
From the video transcript, we find that:
- (A) is Sodium Metaborate (NaBO2).
- (B) is Sodium Tetraborate (Na2B4O7).

3. **Write the Reaction for (A)**:
Now we have:
\[ (A) + \text{CO}_2 \rightarrow (B) + \text{Na}_2\text{CO}_3 \]
Substituting (A) into the equation, we get:
\[ \text{NaBO}_2 + \text{CO}_2 \rightarrow \text{Na}_2\text{B}_4\text{O}_7 + \text{Na}_2\text{CO}_3 \]
Here, (B) is confirmed as Sodium Tetraborate (Na2B4O7).

4. **Continue with the Next Reaction**:
The next reaction involves (B) reacting with concentrated HCl:
\[ (B) + \text{conc HCl} \rightarrow \text{NaCl} + \text{Acid} (C) \]
Here, (C) is likely Boric Acid (H3BO3).

5. **React (C) with Water**:
The next step is:
\[ (C) + \text{H}_2\text{O} \rightarrow \text{Acid} (D) \]
This indicates that (D) is likely Boric Acid in a different form or hydrated.

6. **Strong Heating of (D)**:
The next reaction is:
\[ (D) \xrightarrow{\text{Strong heating}} (E) \]
This could lead to the formation of Boron Oxide (B2O3).

7. **Final Reaction with CuSO4**:
The last reaction is:
\[ (E) + \text{CuSO}_4 \xrightarrow{\text{Heated}} \text{Blue-coloured compound} (F) \]
This blue compound is likely to be Copper(II) Borate or a similar complex.

### Conclusion:
From the analysis, we find that the compound (B) is **Sodium Tetraborate (Na2B4O7)**.