Looking at the given diagram, 
PQ is parallel to RS
This means that 
Triangle PQT is similar to triangle RST
We can say that 
PT/RT = QT/ST
Given that 
PT = 30 cm, QT = 60 cm
RP = 5 cm 
RT = RP + PT = 5 + 30
RT = 35 cm
ST = QT + SQ = 60 + SQ
Therefore, 
30/35 = 60/(60 + SQ)
By cross multiplying, it becomes
30(60 + SQ) = 35 * 60
1800 + 30SQ = 2100
30SQ = 2100 - 1800
30SQ = 300
SQ = 300/30
SQ = 10 cm
OPtion A is Correct