Hi,
Really, I think ' is the main clause, and '(that) these foods are not soft enough for me' is a subordinate noun clause. We can see that clearly in option (c).
1. These foods are not soft enough for me, I think. This word order makes it look like two clauses. The 'I think' seems parenthetical, and the comma acknowledges that.
2. I think, these foods are not soft enough for me. There is not reason here to use a comma.
3 I think these foods are not soft enough for me. As discussed.
Each sentence contains two main clauses.How can we tell they are arranged grammatically ?
'These foods' suggests you are focusing on different kinds of food. If not, just say 'This food'.
Best wishes, Clive