According to scientists, gigantic explosion by which universe came into existence is called:
a. universal theory
b. solar system theory
c. milky way theory
d. big bang theory

Hint: When you attempt questions based on the theories of creation of the universe keep in mind the various theories and who gave such theories and how they came to be. Like in this question the gigantic explosion is hinting towards the big band theory.

Complete step-by-step answer:
When we talk about the creation of the universe, several theories come into mind. Like the Big Bang theory, the universal theory, cosmic theory, milky way theory etc.
The most widely accepted theory is that of the Big Bang theory. This theory was born of the observation that other galaxies are moving away from our own at great speed in all directions, as if they had all been propelled by an ancient explosive force.
A Belgian priest named Georges Lemaître first suggested the big bang theory in the 1920s, when he theorized that the universe began from a single primordial atom. The idea received major boosts from Edwin Hubble's observations that galaxies are speeding away from us in all directions, as well as from the 1960s discovery of cosmic microwave radiation—interpreted as echoes of the big bang—by Arno Penzias and Robert Wilson.
Another theory we know about Is the Modern Laplacian Theory or more commonly known as the solar system theory, which French astronomer and mathematician Pierre-Simon Laplace first suggested in 1796 that the Sun and the planets formed in a rotating nebula which cooled and collapsed. The theory argued that this nebula condensed into rings, which eventually formed the planets and a central mass - the Sun. The slow spin of the Sun could not be explained.
The modern version assumes that the central condensation contains solid dust grains which create drag in the gas as the center condenses. Eventually, after the core has been slowed, its temperature rises and the dust evaporates. The slowly rotating core becomes the Sun. The planets form from the faster rotating cloud.
There are many other theories which discuss the creation of the universe, but the gigantic explosion is a clear giveaway that the theory being talked about is the Big Bang theory.

So, the correct answer is “Option d”.

Note: There are several theories which tell us about the creation of the Milky Way galaxy, a spiral disk galaxy which is similar to many others we see in the sky, but how the universe made the Milky Way from such simple beginnings is not altogether clear. Many astronomers attempt to construct models to explain the same.