(a)Concave mirror of focal length 10 cm

(b)Convex mirror of focal length 10 cm

(c)Concave mirror of focal length 3.75 cm

(d)Convxe mirror of focal length 2.5 cm

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Since the focal length of equiconvex of $\mu =1.5 \;cm$

$|R_1 |=|R_2|=f=10\;cm$

Power of the combination

$P=2 P_L+P_M$

$-\large\frac{1}{F} =2( \mu-1) \bigg[ \large\frac{1}{R_1} -\frac{1}{R_2}\bigg]-\frac{2}{R_2 }$

$\qquad= 2(1.5 -1) \bigg[ \large\frac{1}{15} -\frac{1}{-15}\bigg] -\frac{2}{-15}$

$\qquad= \large\frac{-1}{F}=\frac{4}{15}$

$F= -\large\frac{-15}{14}$

$\quad= -3.75\;cm$

System behave like concave mirror of focal length $3.75\;cm$

Hence c is the correct answer.

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