The total number of elements present in the 4th period of the modern periodic table is:(A) 8(B) 18(C) 32(D) 12

To determine the total number of elements in the 4th period of the modern periodic table, we can refer to the structure of the periodic table and the arrangement of electrons in atomic orbitals.

### Explanation:

1. **Periods in the Periodic Table**:
- The periodic table is organized into rows called periods. Each period corresponds to the filling of a principal energy level (n) of electrons.

2. **4th Period**:
- The 4th period corresponds to the filling of the n=4 energy level.
- The orbitals that can be filled in this period are:
- 4s
- 3d
- 4p

3. **Filling Order**:
- According to the Aufbau principle, the order of filling orbitals is as follows:
- The 4s orbital is filled first, followed by the 3d and then the 4p orbitals.
- The maximum number of electrons that can occupy each type of orbital is:
- 4s: 2 electrons
- 3d: 10 electrons
- 4p: 6 electrons

4. **Total Elements**:
- Therefore, the total number of electrons (and hence the total number of elements) in the 4th period is:
\[
2 \, (\text{from } 4s) + 10 \, (\text{from } 3d) + 6 \, (\text{from } 4p) = 18 \, \text{electrons}
\]

### Conclusion:
Thus, the total number of elements present in the 4th period of the modern periodic table is **(B) 18**.