Which of the following trivalent ion has the largest ionic radii in the lanthanide series?

The correct option is A La
In Lanthanide series, size decreases with increase in atomic number.
Due to imperfect shielding of one electron by another in the same sub-shell is the cause for lanthanoid contraction. Due to lanthanoid contraction the size of the atoms of lanthanoid series decrease with increase of atomic number.

Note: Lanthanoids series is from Ce to Lu; Lanthanum closely resembles the lanthanoids so it included in discussion of lanthanoids.

So, due to lanthanide contraction, reduction in size from $L{a}^{3+}$ to $L{u}^{3+}$ is observed.
Thus, $L{a}^{3+}$ has largest size among all options.