Can somebody please help me with this question?

Explanation:

I believe the answer would be:

Te2- > I- > Cs+ > Ba2+ > La3+ (from biggest to smallest)

This is what you wrote above.  This makes sense because all 5 ions are isoelectronic with xenon (i.e. they all have 54 electrons) but the number of protons (positively charged particles in the nucleus of the atom) of each is different:

La3+ has 54 electrons and 57 protons, meaning that there are 3 additional positive charges to attract the negatively-charged electrons, bringing them closer in towards the nucleus (a "stronger" grip, and therefore, smaller-sized ion if you can imagine).  

Ba2+ has 54 electrons but 56 electrons (only 2 additional positive charges).  

Cs+ has 54 electrons and only 55 protons (only 1 additional positive charge).

I- has 54 electrons and only 53 protons (1 fewer positive charge).

Te2- has 54 electrons and only 52 protons (2 fewer positive charges).

Anions (negatively charged ions) that are isoelectronic with a particular noble gas (in this case, xenon) are generally larger in size than cations (positively charged ions) that are also isoelectronic with the same noble gas.