Hi Cheese12--

Here's how we look at it - EssaySnark's explanations are never the official grammarian rules, we just try to show how we think about things.

In this case, "having" is correct because it indicates an ongoing state. It's a current, active verb.

Another very clumsy (but grammatically correct) way to rephrase this sentence would be:

Polydactyly is what having more than the usual number of fingers or toes is called.

(We're substituting "is called" there instead of "is termed" for this exercise, since it's simpler and means the same.)

In other words, "polydactyly" is what THE STATE or THE CONDITION of having blah blah blah is called.

You wouldn't say "Polydactyly is what to have blah blah blah is called."

EssaySnark is not claiming that this type of sentence transformation/test would work in all instances, but it does here.

No idea if this is helpful but hopefully it is!!!

EssaySnark

IMO: C

Having more than the usual numbers of fingers or toes on the hands or feet is termed polydactyly.

A) Having more than the usual numbers of fingers or toes
B) Having had more than the usual number of fingers or toes
C) Having more than the usual number of fingers or toes
D) To have more than the usual number of fingers or toes
E) To have more than the usual numbers of fingers or toes

In general, if we need a verb, the infinitive is preferable. If we're introducing a modifying clause then the participle is preferable. If we are using a verb that ends in -ing as a NOUN, that is called a gerund.

There's a very nice lesson on Gerunds, Participles and Infinitives here: https://owl.english.purdue.edu/owl/resource/627/02/

The answer is C here because "Having more" is used as the subject, and is a gerund.