Step 1: Read the sentence.
He always ____ to prove that the earth revolves around the sun.
Step 2: Spot the key signal.
The adverb always marks a regular habit or timeless truth, not a one-time event.
Step 3: Recall the matching tense.
Habits and permanent facts take the simple present tense.
Step 4: Apply subject agreement.
The subject He is third person singular, so the verb needs an s ending, giving tries.
Step 5: Reject the others.
tried is past, was trying is past continuous, and is trying is a temporary present action; none suit a permanent habit.
Step 6: Conclude.
The habitual present form is tries, option B.
\[ \boxed{\text{B. tries}} \]