@rama @praveen sir and @ayushi
Yes, Here we consider all 32 chips are in a single row .
Each row of memory module gets refreshed at once and with each cycle we can refresh 1 row of each chip at once.
- For a refresh, only the row address is needed, so a column address doesn't have to be applied to the chip address circuits. 
- During a memory refresh cycle, all memory chips are enabled so that memory refresh is performed on every chip in the memory module simultaneously .
Also see the problem specified in Memory Refresh Period in 2nd Reference . There are 4 * 8= 32 dynamic RAM's but as all dynamic RAM's are placed in a single row so they consider only number of rows ( that is 64 ) while calculating the memory time spent on refresh. [ Almost same as this question ]
Also, option D i.e. 3200×220 can not be the answer, as 1K row = 210 rows , so when for one chip the total time taken for refresh 100 * 210 for 32 chips it would be 32 * 100 * 210 = 3200 * 210 which NOT in option also . In option we have 3200 * 220 so Option D can not be the answer ..
Hence B is the correct Answer .