Refer to the given dichotomous key and select the correct statement.
(i) (a) Single parent is involved in reproduction.-Go to (ii)
(b) Two parents arc involved in rcproduction.-Go to (iii)
(ii) (a) The nucleus of the parent body divides only once. - P
(b) The nucleus of the parent body divides repeatedly - Q
(iii) (a) Fertilisation occurs outside parent body. - R
(b) Fertilisation occurs inside parent body. - Go to (iv)
(iv) (a) Embryonic development occurs outside parent body. - S
(b) Embryonic development occurs inside parent body. T

Options

A. P occurs in Plasmodium while Q occurs in Amoeba.
B. In S, both the types of gametes are released from the body while in T, only one type of gamete is released from the body.
C. R involves only mitosis while S involves both mitosis and meiosis.
D. In S, eggs arc surrounded by protective calcareous shell while in T protective calcareous shell is absent around the eggs.

Explanation

To solve the question using the provided dichotomous key, we will follow the steps outlined in the key to determine the correct statement.

### Step-by-Step Solution:

1. **Identify the type of reproduction**:
- The first question in the dichotomous key is whether a single parent or two parents are involved in reproduction.
- If **(a)** Single parent is involved, we go to step (ii).
- If **(b)** Two parents are involved, we go to step (iii).

2. **Determine the type of division**:
- If we go to step (ii) (single parent):
- **(a)** If the nucleus of the parent body divides only once, we identify it as **P** (binary fission).
- **(b)** If the nucleus divides repeatedly, we identify it as **Q** (multiple fission).
- If we go to step (iii) (two parents):
- **(a)** If fertilization occurs outside the parent body, we identify it as **R** (external fertilization).
- **(b)** If fertilization occurs inside the parent body, we go to step (iv).

3. **Identify embryonic development**:
- If we go to step (iv):
- **(a)** If embryonic development occurs outside the parent body, we identify it as **S** (oviparous).
- **(b)** If embryonic development occurs inside the parent body, we identify it as **T** (viviparous).

4. **Analyze the statements**:
- Based on the definitions:
- **P** corresponds to organisms that reproduce by binary fission.
- **Q** corresponds to organisms that reproduce by multiple fission.
- **R** corresponds to organisms that undergo external fertilization.
- **S** corresponds to oviparous organisms (egg-laying).
- **T** corresponds to viviparous organisms (live-bearing).

5. **Select the correct statement**:
- The correct statement from the options provided is:
- **S**: "Eggs are surrounded by protective calcareous shell" (correct for oviparous).
- **T**: "Calcareous shell is absent around the egg" (correct for viviparous).
- Therefore, the statement regarding S and T is correct.

### Final Answer:
The correct statement is that in **S**, eggs are surrounded by a protective calcareous shell, while in **T**, the calcareous shell is absent around the egg.