The electropositive character is the maximum for which of the following elements in Group $13$ ? 

Electropositive character of an element is directly dependent on its size. Bigger sized elements have more electropositive character.

Correct option :(1) Due to lanthanide contraction, the trend of ionic radii in group $13$ atoms follows the order:

$B<Al>Ga<In<Tl$ 

The Aluminum has the biggest ionic radii. Therefore, aluminum is the most electropositive element in group $13.$

Hence, this option is correct.

Incorrect options :

(2) As it is evident from the order of atomic radii given above, the atomic radii or Thallium is second biggest in group $13$ elements with aluminum being the biggest element. Hence, the most electropositive element in group $13$ is aluminum and not Thallium. Hence, this option is not correct.

(3) Boron is the smallest sized element of group $13.$ Hence, we can say that it is the least electropositive element of group $13$. Hence, this option is incorrect.

(4) It can be easily inferred from the atomic radii order given above that Indium is not the biggest sized element in group $13$ but Aluminum is. Hence, the most electropositive element in group $13$ is Aluminum and not Indium. Hence, this option is incorrect.

The electropositive character is maximum for Aluminum (Al) in group $13$.

 Hence the correct option is (1) .