Refer to the given dichotomous key.
I (a) It is a mineral nutrient. - Go to II
(b) It is a vitamin. - Go to III
II (a) It is a constituent of haemoglobin. - P
(b) It is a constituent of thyroxine hormone. Q
III (a) It helps in clotting of blood. - R
(b) It helps in building strong bones and teeth. - S
Identify P, Q, R, and S and select the correct option regarding them

Options

A. Deficiency of P causes a disorder called anaemia which can also be caused by deficiency of a vitamin known as `B_(12)`.
B. Our body is capable of manufacturing Q so we need not supplement it externally through our diet.
C. Deficiency of R causes beriberi in children and pellagra in adults.
D. S also helps in building resistance against infection in children and is found abundantly in citrus fruits.

Explanation

To solve the question using the given dichotomous key, we will identify the substances P, Q, R, and S step by step:

### Step 1: Identify P
- The key states: "It is a mineral nutrient."
- We go to II, where it states: "It is a constituent of haemoglobin."
- The mineral that is a constituent of haemoglobin is **Iron (Fe)**.
- Therefore, **P = Iron**.

### Step 2: Identify Q
- Continuing from II, the next option states: "It is a constituent of thyroxine hormone."
- The mineral that is a constituent of the thyroxine hormone (produced by the thyroid gland) is **Iodine (I)**.
- Therefore, **Q = Iodine**.

### Step 3: Identify R
- Now we go to III, which states: "It is a vitamin."
- The first option under III states: "It helps in clotting of blood."
- The vitamin that helps in blood clotting is **Vitamin K**.
- Therefore, **R = Vitamin K**.

### Step 4: Identify S
- The next option under III states: "It helps in building strong bones and teeth."
- The vitamin that is essential for building strong bones and teeth is **Vitamin D**.
- Therefore, **S = Vitamin D**.

### Summary of Identifications:
- **P = Iron**
- **Q = Iodine**
- **R = Vitamin K**
- **S = Vitamin D**

### Correct Option Regarding Them:
- The question asks to select the correct option regarding P, Q, R, and S.
- The correct option is that deficiency of P (Iron) leads to anemia, deficiency of Q (Iodine) leads to thyroid problems, deficiency of R (Vitamin K) affects blood clotting, and deficiency of S (Vitamin D) affects bone health.