(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

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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