It can't be A because the sentence would have been "Bien qu'il soit intelligent" (words in that order and nothing else before the coma). "Bien que" means "even though". C is the right answer "si [quality] soit-il" is not as uncommon as the other 2 have said. It's a particular turn of phrase that just need to be learn by heart, I don't think there is a grammar rule for it. It might have been created by one of the French Master of literature once. It means "so [quality] he is [it's still not enough]" or if you prefer "he may be really [quality] but..."

Contrary to what Renaud99 says, the answer C is the right one. "Si" means "aussi" here (as intelligent as he is) it is what we call a concessive proposition with a meaning of intensity. We conceed that he is very intelligent but he doesn't have the skills to get this position. Si intelligent soit-il or Bien qu'il soit intelligent have almost the same meaning. This kind of expression is extremly rare. It's quite literary and to be honest I find it quite pompous.

Merci à tous!

This exercice is bad! All the answers are incorrect. The correct answer should be "Aussi". Aussi intelligent soit-il, il manque de compétence pour ce poste. With the first answer the sentence should be Bien qu'il soit intelligent, il manque de compétence pour ce poste. C and D are not usable here.

Moi, j'aurai choisi la réponse "A. bien que" parce que le subjonctif est dans la phrase. Comme vous, si la réponse est vraiment "C. Si" j'aimerais savoir pourquoi? In French, "si" can also mean "as" so I think in this phrase they are saying "As intelligent as he is, he doesn't have the skills for this job". J'espère que quelqu'un peut éclairer davantage sur la question!