Correct answer:

• c1-h3

Black had 2 possibilities that could not be countered without creating another possibility: c7-c5 and e8. With c1-h3 White blocked these two ways out with one single move. Some of the possible Black’s attempts and the respective counters:

• if c7-c5, then i2-g4;x
• if e8, then a4-d4
• if a1-c3, then a3-c5
• if i1-e5, then f8-f2
• if g6, then i4-g4
• if i4-g4, then c7-c5;x

h3 would not have worked. It blocks the c1-i3 line, which gives Black another chance: a4-d4 — and a sure capture on either the d1-d8 line or the b1-i4 line.