PQR is a right angled triangle made of brass rod bent as shown. If it is heated to a high temperature the angle PQR.

Suppose, PQ = a, QR = b and PR = c.It t be the rise in temperature, then a' = a(1+αt), b' = b(1+αt) and c' = c(1+αt).Now, (a')² + (b')²= {a(1+αt)}²+{b(1+αt)}²= (a²+b²)(1+αt)²= c²(1+αt)² = (c')² So           option (3) is right.