Okay, let's get you your answer.
First: No matter what people will tell you, the center of the Earth is hot, hot, hot! So, remove any choices with the "Low," option first.
That leaves us with two choices:
A: high, low
D: high, high
Since we already know that the temperature is high, we can think about how it would sound in the sentence above.
Using answer choice A:
"The high temperature and the low pressure on Earth's inner core force it to remain solid."
Woah there.
Let's think about that for a second. Let's say you put a nickle in a place where the temperature is at the metal's melting point without pressure, or anything that is forced upon it to take shape. There is no way that little nickle would stay solid, am I correct? So, answer choice A, is inncorrect.
Using answer choice D:
"The high temperature and the high pressure on Earth's inner core force it to remain solid."
There we go!
Once read over, you will find that you can point out how pressure is able to have a factor on keeping something solid when it should be liquified due to temperature!
Your answer choice is D!
I hope this helps you! Have a good day!