[SunilNagpal]

Q: Which amino acid has three ionizable sites?
A. Glycine
B. Leucine
C. Valine
D. Lysine

Ans: Lysine (I guess)

Following is a list of pKa values for the a-carboxylic acid group, the a-amino group, and any ionizable side chains (the ionizable groups in the same order).

Alanine
2.35
9.87

Arginine
2.01
9.04
12.48

Asparagine
2.02
8.80

Aspartic Acid
2.10
9.82
3.86

Cysteine
2.05
10.25
8.00

Glutamic Acid
2.10
9.47
4.07

Glutamine
2.17
9.13

Glycine
2.35
9.78

Histidine
1.77
9.18
6.10

Isoleucine
2.32
9.76

Leucine
2.33
9.74

Lysine
2.18
8.95
10.53

Methionine
2.28
9.21

Phenylalanine
2.58
9.24

Proline
2.00
10.60

Serine
2.21
9.15

Threonine
2.09
9.10

Tryptophan
2.38
9.39

Tyrosine
2.20
9.11
10.07

Valine
2.29
9.72

Lysine is the correct one.