Which external trait determines sex correctly?
(a)Female Cockroach-Anal cerci.
(b)Male Shark-Claspers on pelvic fins.
(c)Female Ascaris-Curved posterior end.
(d) Male Frog-Copulatory pad on the first digit of the hindlimb.

Hint: These are paired intromittent organs which are modifications of some structure present at the inner margin of that area located near the cloaca and are used for transferring the sperms to the female genital tract.

Complete step by step answer:
The external trait that determines sex correctly is male shark-claspers on pelvic fins.
The male sharks are similar in appearance but anatomically smaller than the females. But the easiest way to recognize them is to look for claspers that are present on the inner margin of pelvic fins in the male sharks. Male sharks bear a pair of claspers, which are used during mating to deliver sperm inside of a female shark.

So, the correct answer is 'Male Shark- Claspers on pelvic fins'.

Additional information: -The claspers are like a pair of extra, roll-up fins under their body which is similar to a penis.
-The claspers are an external appendage found on male sharks, skates, and rays that are designed to deliver sperm inside of a female. The Grooves within the claspers help to channelize the semen where it needed to go. However, they are not only similar but dissimilar also to a penis so they are not an independent appendage.
-These claspers are deeply grooved cartilaginous extensions of the shark's pelvic fins.
-The body of male Ascaris are having a curled posterior end, whereas the females are having a straight posterior end.
-Male frogs can be recognized or distinguished from the female frog by the presence of sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs only, not the hind limbs. These are absent in female frogs.
-Anal cerci are a pair of appendages found at the end of the abdomen and this appendage is arising from near the 10th segment of the cockroach body. These are present in both males and females. The anal style appendages are present in males that are absent in females.

Note:
-The male sharks are similar in appearance but anatomically smaller than the females.
-Male sharks bear a pair of claspers, which are used during mating to deliver sperm inside of a female shark.
-The claspers are like a pair of extra, roll-up fins under their body which is similar to a penis.
-The Grooves within the claspers help to channelize the semen where it needed to go.