You look at the electronegativities of the atoms in the bonds and then calculate the electronegativity differences #ΔEN#.
Method 1. A quantitative method
Here are the electronegativities of the atoms.
#bb(color(white)(mmml)"Sr"color(white)(mm)"Sn"color(white)(mll)"Si"color(white)(mm)"Cl")#
#ENcolor(white)(ml)0.95color(white)(ml)1.90color(white)(ml)1.96color(white)(ml)3.16#
Now, we calculate the electronegativity differences for the #"M-Cl"# bonds.
#bb(color(white)(mmmmmmmmmll)ΔEN)#
#"Sr-Cl"color(white)(ml)"|0.95 - 3.16| = 2.21"#
#"Sn-Cl"color(white)(m)"|1.90 - 3.16| = 1.26"#
#"Si-Cl"color(white)(ml)"|1.96 - 3.16| = 1.20"#
#"Sn"# is a metal. It has metallic bonds.
The bond that has the smallest electronegativity difference will have the greatest covalent character.
The #"Si-Cl"# bond has the smallest difference, so it has the greatest covalent character.
Method 2. A qualitative method
#"Sn"# is a metal. It has metallic bonds, not covalent bonds.
#"Sn"# and #"Sr"# are both metals. Their bonds with the nonmetal #"Cl"# will have mostly ionic character.
#"Si"# is a metalloid. It is closest to #"Cl"# in the Periodic Table, so its bonds to #"Cl"# should have the most covalent character.
