Full m-ary tree means : Every internal node have exactly m children. Other way, each node has zero or m children only.
Some of the full m-ary trees.
T1 (m=2) , T2 (m=3) , T3 (m=5) are full but T_{4 }is not.
In this question, how they define complete means the full m-ary tree. We have n-ary tree and Internal nodes ( I ) = 10 , Leaf nodes ( L ) = 41
$n = L+ I \\ =>n = 51 \\ =>n*10+1 = 51 \\ =>n = 5$