Not just (c) but even option (b) is also true.
Thrashing also decreases the degree of multiprogramming because as per the definition of degree of multiprogramming it states that more no of pages to numerous processes are made to be stored at the main memory(ram) so as to asure the best cpu utilization. But thrashing occurs when cpu tends to execute a particular process and seeks for it's respective page in that instance the first page is present but second is absent in the main memory so inorder a hit fails... It now needs to search for that page in secondary memory and need to bring that page from secondary memory to the main memory for it's complete execution (page fault) .. In that case the degree of multiprogramming decreases.
And while bringing those pages... Consider a situation where multiple page faults occurs ie. The pages required are not available in the main memory then cpu will invest more of it's time in bringing the pages from secondary memory to main memory for it's execution. Due to which excessive page i/o occurs...
Hence (b) and (c) both the options are correct.
Hope this helps.