Due to the intense heat and pressure on the inner core, scientists believe it is a a. solid b. liquid c. semi-fluid d. gas

A. solid    

Explanation:

There is intense pressure and heat on the inner core. With so much heat, it should be liquid but the pressure over rides the heat effect and scientists believe that the inner core is solid. The inner core mainly consists of iron. With increase in pressure, the melting point of Iron also increases. Thus, in spite so intense heat, the inner core is solid while the outer core is liquid.

The correct option is A.