The correct answer is Borneo, the third-largest island in the world, which is politically divided among three countries: Indonesia, Malaysia, and Brunei. Indonesia controls the largest portion of the island, known as Kalimantan, while Malaysia controls the northern states of Sabah and Sarawak, and Brunei occupies a small enclave along the northern coast. The Indonesia-Malaysia land border on Borneo stretches roughly 2,000 kilometres through dense rainforest terrain and is one of the longest land boundaries in Southeast Asia, historically significant due to territorial and resource disputes in border regions. This border is frequently tested because most people associate Indonesia and Malaysia primarily as maritime neighbours across the Strait of Malacca and South China Sea, forgetting that a substantial land frontier also exists on Borneo. Sumatra is incorrect because it lies entirely within Indonesian territory and does not border Malaysia by land, though it faces Peninsular Malaysia across the Strait of Malacca. Java, Indonesia's most populous island and home to Jakarta, is also entirely Indonesian and has no shared land border with any foreign country. Sulawesi, an Indonesian island known for its unique K-shaped geography, is likewise wholly within Indonesia and does not touch Malaysian territory. A common exam confusion is assuming Indonesia and Malaysia are separated purely by sea because Peninsular Malaysia lies across a strait from Sumatra, leading students to overlook the East Malaysia-Kalimantan land connection. Another related confusion is forgetting that Indonesia also shares land borders with Papua New Guinea on New Guinea island and Timor-Leste on Timor island, making Borneo just one of three islands where Indonesia has land neighbours. Revision takeaway: Indonesia's three land-bordering neighbours are Malaysia (on Borneo), Papua New Guinea (on New Guinea), and Timor-Leste (on Timor).
