This question asks about a world record held by the plant Rafflesia arnoldii.
Tallest cacti: Rafflesia is not a cactus at all, it is a parasitic flowering plant with no visible leaves, stem, or roots of its own, so this option does not fit.
Largest bloom: Rafflesia arnoldii produces the single largest individual flower in the plant kingdom, some blooms grow past 3 feet (about 1 metre) wide and can weigh up to 10 kg.
Longest surviving tree: Rafflesia is not a tree, and this record belongs to species like the bristlecone pine, so this is wrong.
Smallest plant: The smallest flowering plant title goes to duckweed species, not Rafflesia, so this is wrong too.
So option B, largest bloom, is correct.
Let's summarize:
Rafflesia arnoldii holds the record for the single largest flower on Earth.
Knowing which plant holds which record helps clear such general knowledge picks fast.