The correct answer is (a) Kew (England).
Kew Gardens is a botanical garden in southwest London that houses the largest and most comprehensive collection of plant species in the world. It was founded in 1759, and today it covers 120 hectares (300 acres) of land. The gardens are home to over 30,000 species of plants, including many rare and endangered species. They are also home to a number of important research facilities, and they are a popular tourist destination.
The other options are incorrect.
(b) Paris (France) is home to the Jardin des Plantes, which is a botanical garden and natural history museum. It is the oldest botanical garden in France, and it is one of the largest in Europe. However, it is not the largest botanical garden in the world.
(c) Howrah (India) is home to the Indian Botanic Garden, which is a botanical garden and research institute. It is the oldest botanical garden in India, and it is one of the largest in Asia. However, it is not the largest botanical garden in the world.
(d) Tokyo (Japan) is home to the National Museum of Nature and Science, which has a botanical garden attached to it. However, the botanical garden is not the largest in the world.