(A)

[tex] {a}^{2} = {b}^{2} + {c}^{2} [/tex]

[tex] {a}^{2} = {28}^{2} + {21}^{2} [/tex]

[tex] {a}^{2} = 784 + 441[/tex]

[tex] {a}^{2} = 1225[/tex]

[tex] \sqrt{1225 = a} [/tex]

[tex]a = 35[/tex]

(B)

[tex] {a}^{2} = {b}^{2} + {c}^{2} [/tex]

[tex] {25}^{2} = {24}^{2} + {c}^{2} [/tex]

[tex]625 - 576 = {c}^{2} [/tex]

[tex]49 = {c}^{2} [/tex]

[tex] \sqrt{49 = c} [/tex]

[tex]7 = c[/tex]

(C)