Ionization enthalpy increases across a period because:

1. Outermost electrons are held more and more tightly:
- Ionization enthalpy is the energy required to remove an electron from an atom or ion in the gaseous state.
- As we move across a period in the periodic table, the atomic number increases, which means the number of protons in the nucleus also increases.
- The increase in the number of protons leads to a stronger positive charge in the nucleus, which attracts the negatively charged electrons more strongly.
- As a result, the outermost electrons in the atoms of elements across a period are held more tightly, requiring more energy to remove them. This increase in the attraction between the nucleus and the outermost electrons leads to an increase in ionization enthalpy.

2. Screening is more effective:
- Effective nuclear charge refers to the net positive charge experienced by an electron in an atom.
- As we move across a period, the number of electrons in the inner shells remains constant, while the number of protons in the nucleus increases.
- The increase in the number of protons increases the positive charge in the nucleus. However, the inner electrons provide some shielding effect, reducing the effective nuclear charge experienced by the outermost electrons.
- This shielding effect decreases as we move across a period because the increase in the number of protons is not completely offset by the increase in the number of inner electrons.
- Therefore, the effective nuclear charge experienced by the outermost electrons increases across a period, making them more tightly held and increasing the ionization enthalpy.

3. Outermost electrons are not held tightly:
- This statement is incorrect. As mentioned earlier, the increase in ionization enthalpy across a period is due to the outermost electrons being held more tightly.

4. Shielding is more effective:
- This statement is incorrect. As mentioned earlier, the shielding effect decreases across a period, leading to an increase in the effective nuclear charge experienced by the outermost electrons.

In summary, ionization enthalpy increases across a period because the outermost electrons are held more and more tightly as the atomic number increases, and the shielding effect becomes less effective.