(i) `P+C("carbon")+Cl_(2) to Q+CO uaarr`
(ii) `Q+H_(2)O to R+HCl`
(iii) `BN+H_(2)O to R+NH_(3) uarr`
(iv) `Q+LiAlH_(4) to S+LiCl+AlCl_(3)`
(v) `S+H_(2) to R+H_(2) uarr`
(vi) `S+NaH to T`
(P,Q,R,S annd T do not represent their chemical symbols)
Q. Compound T is used as a/an:

Options

A. oxidising agent
B. complexing agent
C. bleaching agent
D. reducing agent

Explanation

To solve the problem step by step, we will analyze each reaction given and identify the compounds P, Q, R, S, and T.

### Step 1: Identify Compound P
The first reaction is:
\[ P + C + Cl_2 \rightarrow Q + CO \]

From the context, we can deduce that:
- **P** is likely to be **B2O3** (Boric oxide).
- When B2O3 reacts with carbon and chlorine, it forms **BCl3** (Boron trichloride) and carbon monoxide.

### Step 2: Identify Compound Q
From the first reaction, we established:
- **Q = BCl3**

### Step 3: Identify Compound R
The second reaction is:
\[ Q + H_2O \rightarrow R + HCl \]

Substituting Q:
\[ BCl3 + H_2O \rightarrow R + HCl \]

This reaction produces:
- **R = H3BO3** (Boric acid).

### Step 4: Confirm Compound R
In the third reaction:
\[ BN + H_2O \rightarrow R + NH_3 \]

Here, BN (Boron nitride) reacts with water to also produce:
- **R = H3BO3** (Boric acid) and ammonia.

### Step 5: Identify Compound S
The fourth reaction is:
\[ Q + LiAlH_4 \rightarrow S + LiCl + AlCl_3 \]

Substituting Q:
\[ BCl3 + LiAlH_4 \rightarrow S + LiCl + AlCl_3 \]

This reaction produces:
- **S = B2H6** (Diborane).

### Step 6: Confirm Compound S
The fifth reaction is:
\[ S + H_2 \rightarrow R + H_2 \]

Substituting S:
\[ B2H6 + H_2 \rightarrow R + H_2 \]

This reaction confirms:
- **R = H3BO3** (Boric acid) and hydrogen gas.

### Step 7: Identify Compound T
The sixth reaction is:
\[ S + NaH \rightarrow T \]

Substituting S:
\[ B2H6 + NaH \rightarrow T \]

This reaction produces:
- **T = NaBH4** (Sodium borohydride).

### Conclusion
Now we have identified all compounds:
- **P = B2O3**
- **Q = BCl3**
- **R = H3BO3**
- **S = B2H6**
- **T = NaBH4**

### Final Answer
Compound T (NaBH4) is used as a **reducing agent**.