Question is clearly asking about the levels in multi level indexing i.e., B-Tree & B+Tree
As nothing is mentioned about the type of tree, we take it B-Tree by default.
No of records = 20,000
Block Size = 1000B
Key = 40B
Block Pointer = Record Pointer = 10B
n*BP + (n-1)*(Key+RP) <= Block SIze
where n is order of tree
n*10 + (n-1)*50 <= 1000
n<=17.5
Order = 17
No of level = Log (No of records) Base(Order of Tree)
log 20,000 Base17 = 3.49
No of levels = 4
