Pollen grains are generally spherical and measure about:
(a) 25-50 centimetres
(b) 25-50 millimeters
(c) 25-50 micrometers
(d) 25-50 nanometres

Hint: Pollen grains are the male microgametophytes of seed plants. It exists in a wide variety of shapes and sizes. When dry, the shape is generally spherical or oval. The size of the pollen grain is usually in micrometers.

Complete answer:
The male reproductive part of a plant, called the stamen, comprises two parts: Anther and Filament.
The anther is a bilobed, four-sided tetragonal structure consisting of four microsporangia located at the corners, two in each lobe.
A young anther has a group of compactly arranged homogenous cells called sporogenous tissue which is at the center of each microsporangium. These sporogenous tissues undergo meiotic divisions to form microspore tetrads. When the anther matures and dehydrates, microspores dissociate and develop into pollen grains.
A pollen grain represents a male gametophyte. They are generally spherical and measure about 25-50 micrometers in diameter. It is divided into two prominent layers, the hard outer layer is called the exine and the inner wall is called the intine.

Additional Information: The smallest pollen grain is found in a plant named Myosotis scorpioides. Its pollen grain measures 5 micrometers in diameter.
Corn pollen grains are large, about 90-100 micrometers.
Grass pollen grains are 20-25 micrometers.
Oenothera biennis is known to have the largest pollen grain.
Zostera is a plant that has pollen grains about 2500 micrometers long. This plant is known to have the longest pollen grain till now.
So, the correct answer is ‘(c) 25-50 micrometers’.

Note: Although there are certain species of plants that have pollen grains that are of different sizes, some are quite small while some are quite large the average size of the pollen grains is in a range of 25 to 50 micrometers.