—What’s the matter with you? You look so worried.
-- I don’t know ___________ the problem.

--What’s the matter with you? You look so worried.

--I don’t know ___________ the problem.

1. A.
   what to solve
2. B.
   how to solve
3. C.
   to solve what
4. D.
   how can I

--- What’s the matter with you? You look so worried.
--- I don’t know ___________ the problem

1. A.
   what to solve
2. B.
   how to solve
3. C.
   to solve what
4. D.
   how can I

1 

—What’s the matter with you? You look so worried.

-- I don’t know ___________ the problem.