Computing number of block reads given relational algebra statement
select * from R1 Where B=1

You don't have any index on a search field (B), hence you have to do a full table scan. It means that you fetch all relation's blocks one by one and take the records which satisfy the condition B=1. (Cost - 200000/200 = 200 blocks)

select * from R2 Where C=1

There is not enough information - you have to know(at least approximately) how many records in R2 satisfy the condition C=1.
For example in case that all the records in R2 have C=1 you'll do the full table scan like above (cost - 45000/4500 = 10 blocks) 
On the other hand if you have only one record with C=1 you'll read 3 blocks of index and another one from disk(cost 3 + 1 = 4 blocks)