The atomic size of Ga and Al are almost same because of:
Poor shielding effect of d−orbitals electrons in Ga
More shielding effect of d−electrons in Ga
Poor shielding effect of f−orbital in Ga
High shielding effect of 3s, 3p and 3d electrons in Ga
The correct option is A Poor shielding effect of d−orbitals electrons in Ga
Penetration describes the proximity of electrons in an orbital to the nucleus. Electrons that have greater penetration can get closer to the nucleus and effectively block out the charge from electrons that have less proximity. This is known as shielding.
The shielding effect of d-electrons are poor and this leads to smaller atomic radii than expected. Hence, Ga even though lies below Al, has almost same atomic size.