Step 1: The word 'Although' at the start of the sentence signals a contrast between the initial reaction to the hypothesis and what happened afterward, shown by 'subsequent experiments'.
Step 2: For a contrast to exist, the two blanks must describe opposite or shifting attitudes. Check 'enthusiasm ... weak': enthusiasm is already positive, so there is no negative-to-positive shift, this fails the contrast test. Check 'approval ... compelling': approval is already positive, so again no shift.
Step 3: Between the remaining two, 'skepticism ... contradictory' does show a shift, but 'contradictory evidence' would not really support the hypothesis, it would confuse it. 'Skepticism ... compelling' shows doubt turning into strong supportive proof, which best matches a hypothesis that succeeds after being doubted.
Correct option: (2) skepticism ... compelling.